Exposure method for overlaying one mask pattern on another

ABSTRACT

An exposure method in which mask patterns are overlaid on one another on a substrate, which is an object to be exposed, by using a first and second exposure apparatuses having respective exposure fields of different sizes. The exposure method includes the steps of: sequentially transferring a first mask pattern onto the substrate in the form of a first array in units of a shot area of a predetermined size by using the first exposure apparatus; detecting at least either one of a perpendicularity error of the first array from a design value and a mean value of rotation angles of the shot areas in the first array when a second mask pattern is to be sequentially transferred onto the substrate in the form of a second array in units of a shot area different in size from the unit shot area of a predetermined size by using the second exposure apparatus; and rotating the second mask pattern and the substrate relative to each other through an angle corresponding to a result of the detection, and thereafter, sequentially transferring the second mask pattern onto the substrate.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an exposure method fortransferring a mask pattern onto a photosensitive substrate duringphotolithography processes in the manufacture of semiconductor devices,liquid crystal display devices, imaging devices (e.g. CCD), thin-filmmagnetic heads, etc. More particularly, the present invention relates toan exposure method which is suitably applied to a process in whichexposure is sequentially carried out by the mix-and-match method withrespect to two layers, that is, a layer called “middle layer”, whichrequires no high resolution, such as an ion-implanted layer used inproduction of a semiconductor memory or the like, and a layer called“critical layer”, which requires high resolution.

[0003] 2. Related Background Art

[0004] Exposure apparatuses, e.g. step-and-repeat reduction projectiontype exposure apparatuses (steppers), are used in photolithographyprocesses for producing semiconductor devices, liquid crystal displaydevices, etc. Generally, a semiconductor device such as a VLSI is formedby stacking a multiplicity of pattern layers on a wafer while effectingalignment for each layer. Among the pattern layers, a layer that needsthe highest resolution is called “critical layer”, and a layer thatneeds no high resolution, e.g. an ion-implanted layer used in productionof a semiconductor memory or the like, is called “middle layer”. Inother words, the line width of a pattern which is exposed for the middlelayer is wider than the line width of a pattern exposed for the criticallayer.

[0005] There has been an increasing tendency for recent VLSImanufacturing factories to carry out exposure operations for differentlayers by using respective exposure apparatuses in a process forproducing a single type of VLSI in order to increase the throughput(i.e. the number of wafers processed per unit time) in the productionprocess. Under these circumstances, it has become common practice tocarry out what is called “mix-and-match” exposure. In the mix-and-matchexposure process, exposure for the critical layer is carried out byusing a first stepper of high resolution which performs one-shotexposure with a demagnification ratio of 5:1, and exposure for themiddle layer is carried out by using a second stepper of intermediateresolution which performs one-shot exposure with a demagnification ratioof 2.5:1. In this case, the size of the exposure field of the secondstepper is twice as large as that of the first stepper in bothlengthwise and breadthwise directions, and the throughput of the secondstepper in the exposure process is approximately four times that of thefirst stepper. This will be explained below with reference to FIG. 35.

[0006] Assuming that, as shown in FIG. 35, exposure units on a waferwhich are to be exposed by the first stepper are square shot areas SA₁₁,SA₁₂, SA₁₃, SA₁₄, . . . each surrounded by sides which are parallel toX- and Y-axes perpendicularly intersecting each other, an exposure areawhich is to be exposed by the second stepper is a shot area SB₁ which isso large as to substantially contain the four shot areas SA₁₁ to SA₁₄.When exposure is to be carried out by the second stepper over the fourshot areas SA₁₁, SA₁₂, SA₁₃ and SA₁₄ exposed by the first stepper, thesecond stepper effects alignment of the shot area SB₁, which correspondsto the exposure field of the second stepper, on the basis of alignmentmarks (wafer marks) attached to the shot areas SA₁₁ to SA₁₄.

[0007] There is another conventional exposure method in which, forexample, a step-and-scan type scanning exposure apparatus with ademagnification ratio of 4:1 is combined with either the above-describedfirst or second stepper. The step-and-scan exposure is a process inwhich a shot area on a wafer which is to be exposed is stepped to ascanning start position, and thereafter a reticle, which serves as amask, and the wafer are synchronously scanned with respect to aprojection optical system, thereby sequentially transferring a patternon the reticle onto the shot area. The exposure field of the scanningexposure apparatus is equal, for example, in the width of thenon-scanning direction to the exposure field of the first stepper, butthe exposure field width in the scanning direction of the scanningexposure apparatus is 1.5 times that of the first stepper. It should benoted that there are various combinations of different exposure fieldsizes of a plurality of exposure apparatuses used in the mix-and-matchexposure method in addition to the above-described combinations.

[0008] Thus, the throughput of an exposure process can be increased bycarrying out a mix-and-match exposure process using different exposureapparatuses in combination according to the resolution required for eachlayer on a wafer as described above. However, when exposure apparatuseshaving respective exposure fields of different sizes are used incombination, if a perpendicularity error remains in the array of shotareas (i.e. shot array) of the preceding layer, i.e. if the anglebetween the X- and Y-axes of the shot array deviates from 90°, a givenoverlay error arises. Such a perpendicularity error is due to the factthat the feed directions of the wafer stage driven by motors are notaccurately perpendicular to each other.

[0009] For example, assuming that in FIG. 35 the imaginary straight line23A passing through the centers of the shot areas SA₁₃ and SA₁₄ in thefour shot areas of the preceding layer is parallel to the X-axis, if theangle between the X- and Y-axes of the shot array deviates from 90° byan angle (perpendicularity error) W, the imaginary straight line 24passing through the centers of the shot areas SA₁₁ and SA₁₃ tilts by theperpendicularity error W [rad] relative to the Y-axis. In this case, ifexposure is carried out with the center of the subsequent shot area SB₁aligned with the center 25 of the four shot areas SA₁₁, SA₁₂, SA₁₃ andSA₁₄, which have the perpendicularity error W, a uniform overlay errorΔx arises in the direction X between the pattern in the shot area SB₁and the pattern in each of the shot areas SA₁₁ to SA₁₄ of the precedinglayer. Assuming that the length of each side of the shot area SA₁₁ is L,the overlay error Δx is approximately L·W/2.

[0010] In a case where each shot area of the preceding layer has a shotrotation (chip rotation) also, an overlay error arises which is similarto that in a case where the angle between the X- and Y-axes of the shotarray deviates from 90°.

[0011]FIG. 36 shows the four shot areas SA₁₁ to SA₁₄ in a situationwhere the perpendicularity error of the shot array is zero, but the shotrotation is θ [rad]. Let us assume that the shot rotation θ is of thesame size as the perpendicularity error W in FIG. 35. In the case ofFIG. 36, even if the subsequent shot area SB₁ is exposed by rotating itsimply through an angle corresponding to the shot rotation θ, a uniformoverlay error Δx of the same size as that in the case of FIG. 35 arisesin the direction of the shot rotation between the pattern in the shotarea SB₁ and the pattern in each of the shot areas SA₁₁ to SA₁₄ of thepreceding layer.

[0012] That is, when exposure is sequentially carried out by usingexposure apparatuses having respective exposure fields of differentsizes, if the array of shot areas of the preceding layer has aperpendicularity error or a shot rotation, a uniform overlay errorarises if the subsequent shot areas are simply aligned with respect tothe preceding shot areas.

[0013] On the other hand, in the above-described mix-and-match method,in which after a layer on a wafer has been exposed by a first exposureapparatus, overlay exposure is carried out on the preceding layer byusing a second exposure apparatus, the second exposure apparatus mayeffect alignment by an enhanced global alignment (hereinafter referredto as “EGA”) method as disclosed, for example, in Japanese PatentApplication Unexamined Publication (KOKAI) (hereinafter referred to as“JP(A)”) No. 61-44429 (corresponding to U.S. Pat. No. 4,780,617). Inthis case, however, some problems are experienced, which will beexplained below with reference to FIGS. 37(a) to 38(c).

[0014] FIGS. 37(a), 37(b) and 37(c) illustrate a related art in whichexposure is carried out by the mix-and-match method using two exposureapparatuses having respective exposure fields of the same size. First, apattern image of a reticle RA shown in FIG. 37(b) is transferred ontoeach of shot areas 129A, 129B, . . . , 129I of a first layer, which areshown by the chain lines in FIG. 37(a), on a wafer 20 by using a firstexposure apparatus. In this case, it is assumed that a coordinate systemthat defines each particular travel position of a wafer stage of thefirst exposure apparatus (i.e. stage coordinate system) comprises anX1-axis and a Y1-axis, and that the Y1-axis is tilted by an angle Wclockwise from an ideal Y1*-axis which is perpendicular to the X1-axis.Further, the reticle RA has two identical circuit patterns 112A and 112B(i.e. two-chip pattern) formed in a pattern area 42A. The rotation angleof the reticle RA has been set so that the circuit patterns 112A and112B are arrayed in a direction perpendicular to the X1-axis whenexposure is carried out.

[0015] As a result, the shot areas 129A to 129I of the first layer arearrayed at a predetermined pitch along each of the X1- and X1-axes, andthe shot array has a perpendicularity error W. Further, two identicalcircuit pattern images are transferred onto each of the shot areas 129Ato 129I in such a manner as to lie in side-by-side relation to eachother in a direction perpendicular to the X1-axis.

[0016] Next, a pattern image of a reticle RC shown in FIG. 37(c) istransferred onto each of shot areas of a second layer on the wafer 20 byusing a second exposure apparatus. In this case, it is assumed that astage coordinate system of the second exposure apparatus comprises anX2-axis and a Y2-axis, and that a direction corresponding to the X1-axisof the first layer on the wafer 20 has been set parallel to the X2-axisby pre-alignment carried out in the second exposure apparatus. Althoughthe origins of the coordinate systems (X1,X1) and (X2,Y2) in FIG. 37(a)have been set at the center of the wafer 20 for the sake of explanation,it should be noted that the origins of these coordinate systems may beset at any positions. The reticle RC also has two identical circuitpatterns 127A and 127B formed in a pattern area 42C, and the image ofthe pattern area 42A of the reticle RA as projected on the wafer 20(i.e. exposure field) and the projected image (exposure field) of thepattern area 42C of the reticle RC are of the same size.

[0017] In this case, the second exposure apparatus effects alignment bythe above-described EGA method. That is, array coordinates of wafermarks (not shown) provided for a predetermined number of shot areas(sample shots) selected from the first layer on the wafer 20 aremeasured to thereby calculate array coordinates of all the shot areas inthe stage coordinate system (X2,Y2). Thus, the second exposure apparatuscan recognize that the perpendicularity error W is present in the shotarray on the first layer.

[0018] In the second exposure apparatus, therefore, the rotation angleof the reticle RC is set so that the two circuit patterns 127A and 127Bare arrayed in a direction perpendicular to the X2-axis, as shown inFIG. 37(c), and thereafter, a shot array of a second layer is set bytaking into consideration the perpendicularity error W. Then, exposureis carried out. As a result, the circuit pattern images of the reticleRC are transferred onto each of shot areas 130A, 130B, . . . , 130I ofthe second layer, shown by the solid lines in FIG. 37(a), on the wafer20. Thus, the shot array of the second layer is accurately overlaid onthe shot array of the first layer.

[0019] In a case where the exposure fields (shot areas) of two exposureapparatuses have the same size as described above, even if the shotarray of the first layer has a perpendicularity error, the overlayaccuracy between the first and second layers can be maintained at highlevel by effecting alignment according to the EGA method, for example.

[0020] However, if the shot array of the first layer has aperpendicularity error in a case where the exposure fields of the twoexposure apparatuses have different sizes, the overlay accuracy betweenthe two layers cannot be increased above a certain level by an ordinaryexposure method.

[0021] FIGS. 38(a), 38(b) and 38(c) illustrate a related art in whichexposure is carried out by the mix-and-match method using two exposureapparatuses having respective exposure fields of different sizes. First,a pattern image of a two-chip reticle RA, which has two identicalpatterns 112A and 112B written in a pattern area 42A as shown in FIG.38(b), is transferred onto each of shot areas of a first layer on awafer 20 by using a first exposure apparatus. Next, a pattern image of athree-chip reticle RB, which has three identical circuit patterns 113Ato 113C written in a pattern area 42B as shown in FIG. 38(c), istransferred onto each of shot areas of a second layer on the wafer 20 bya second exposure apparatus. The image of the reticle RB as projected onthe wafer 20 has the same horizontal width as that of the projectedimage of the reticle RA, but the vertical width of the projected imageof the reticle RB is 3/2 times that of the reticle RA.

[0022] In this case also, the stage coordinate system of the firstexposure apparatus is denoted by (X1,Y1), and the stage coordinatesystem of the second exposure apparatus is denoted by (X2,Y2), and it isassumed that alignment and exposure are carried out with the X2-axisaligned with the X1-axis. When exposure is carried out with the firstexposure apparatus by setting the reticle RA so that the two circuitpatterns of the reticle RA are arrayed in a direction perpendicular tothe X1-axis, the circuit patterns are transferred onto each of shotareas 129A, 129B, . . . , 129I of the first layer, shown by the chainlines in FIG. 38(a), on the wafer 20. In this case also, the array ofthe shot areas 129A to 129I has a perpendicularity error in the same wayas in the example shown in FIGS. 37(a) to 37(c).

[0023] Thereafter, the wafer 20 is aligned by the EGA method using asecond exposure apparatus, and then exposure is carried out in such amanner that the three circuit patterns of the reticle RB are arrayed ina direction perpendicular to the X2-axis. Consequently, the threecircuit patterns are transferred onto each of shot areas 131A to 131F ofthe second layer on the wafer 20, as shown by the solid lines in FIG.38(a). However, because each shot area of the first layer has twocircuit patterns transferred thereto, while each shot area of the secondlayer has three circuit patterns transferred thereto, the shot array ofthe first layer and the shot array of the second layer undesirablydiffer from each other in the number of rows in a directionapproximately perpendicular to the X1-axis. As a result, it becomesimpossible to eliminate the effect of a perpendicularity error, which isan error between the rows or columns of a shot array. For example, inFIG. 38(a), if the shot area 129A and the shot area 131A are aligned inthe direction X1 (or X2), a large overlay error arises in the directionX1 between the shot area 129B and the shot area 131A.

[0024] Meanwhile, if both a first and second exposure apparatuses employthe EGA method, the following problems arise. The problems will beexplained below with reference to FIGS. 39(a) to 41(b).

[0025] In this EGA process, array coordinates of a predetermined numberof shot areas (sample shots), which have previously been selected fromamong shot areas on a wafer, are measured to determine, for example, sixcoordinate transformation parameters for calculating array coordinatesin a stage coordinate system, in which the wafer stage is to bepositioned, from the design array coordinates of all the shot areas.

[0026] However, when a pattern for a middle layer is transferred onto acritical layer by the mix-and-match exposure method, for example, agiven overlay error may remain if the coordinate transformationparameters obtained by the EGA method (hereinafter occasionally referredto as “EGA parameters”) are used as they are because differentprojection exposure apparatuses are used for the critical and middlelayers. This means that the EGA parameters may have residual errors. Inorder to correct such residual errors, the conventional practice is tomeasure overlay errors by conducting test printing using marks foroverlay accuracy measurement (hereinafter referred to as “verniermarks”), as described below.

[0027]FIG. 39(a) shows a wafer 20 having vernier marks formed by aprojection exposure apparatus for exposure of a critical layer. In FIG.39(a), shot areas SE1, SE2, . . . , SEM (M is an integer of 12 or more,for example) are arrayed on the wafer 20 at a predetermined pitch alongeach of the X- and Y-axes of an orthogonal coordinate system (X,Y). Ineach shot area SEm (m=1 to M), alignment marks (wafer marks) and overlayaccuracy measuring vernier marks have been formed.

[0028]FIG. 39(b) is an enlarged view showing the mark arrangement in ashot area SEm. In FIG. 39(b), the shot area SEm has a wafer mark 221Xfor the X-axis formed at an end in the direction +Y. The wafer mark 221Xcomprises line-and-space patterns arranged at a predetermined pitch inthe direction X. The shot area SEm further has a wafer mark 221Y for theY-axis formed at an end in the direction +X. The wafer mark 221Ycomprises line-and-space patterns arranged at a predetermined pitch inthe direction Y. The wafer marks 221X and 221Y are marks which aredetected by an imaging detection method (FIA method). Further, the shotarea SEm has vernier marks 222A to 222E formed therein at respectivepositions which are distributed in a cross shape. The vernier marks 222Ato 222E are, for example, box-in-box marks which are detected by animaging detection method (image processing detection method).

[0029] Next, predetermined vernier marks are overlaid on the wafer 20shown in FIG. 39(a) by exposure using a projection exposure apparatusfor a middle layer. For the overlay exposure, it is necessary to obtainarray coordinates of each shot area SEm (m=1 to M) on the wafer 20 inthe stage coordinate system of the projection exposure apparatus for amiddle layer. Therefore, it is assumed that the wafer marks 221A and221Y of each shot area SEm (m=1 to M) indicate the coordinates of thecenter of the corresponding shot area. It is further assumed that thedesign array coordinates of the center of each shot area SEm (m=1 to M)in the coordinate system on the wafer 20 (i.e. the sample coordinatesystem) are (Dxn,Dyn), and that the computational array coordinates ofeach shot area SEm (m=1 to M) in the stage coordinate system of theprojection exposure apparatus for a middle layer are (Fxn,Fyn). In thiscase, the X component Dxn and Y component Dyn of the design arraycoordinates of the center of each shot area SEm are the X coordinate ofthe corresponding wafer mark 221X and the Y coordinate of thecorresponding wafer mark 221Y, respectively, which may be approximatelyexpressed by the following equation (1): $\begin{matrix}{\begin{bmatrix}{Fxn} \\{Fyn}\end{bmatrix} = {{\begin{bmatrix}{Rx} & {- {{Rx}\left( {W + \Theta} \right)}} \\{{Ry} \cdot \Theta} & {Ry}\end{bmatrix}\begin{bmatrix}{Dxn} \\{{Dyn}\quad}\end{bmatrix}} + \begin{bmatrix}{Ox} \\{Oy}\end{bmatrix}}} & (1)\end{matrix}$

[0030] The transformation matrix in Eq. (1) has as elements sixcoordinate transformation parameters (EGA parameters), including scalingparameters Rx and Ry, rotation Θ, perpendicularity W, and offsets Ox andOy. The scaling parameters Rx and Ry are linear expansion andcontraction quantities in the directions X and Y, respectively. Therotation Θ is an angle of rotation of the wafer 20. The perpendicularityW is a perpendicularity error, that is, a deviation of the intersectionangle between the X- and Y-axes from 90°. The offsets Ox and Oy areshift quantities in the directions X and Y, respectively. Next, in orderto determine values of the six coordinate transformation parameters, theprojection exposure apparatus for a middle layer measures arraycoordinates in the stage coordinate system of the wafer marks 221X and221Y provided for each of, for example, 10 shot areas (sample shots)SEa, SEb, SEc, . . . , SEj selected from among the shot areas on thewafer 20 shown in FIG. 39(a). The sample shots SEa to SEj are disposedat the vertices of an approximately regular polygon on the surface ofthe wafer 20 or at uniformly dispersed random positions.

[0031] In this case, the measured values of the array coordinates in thestage coordinate system of the wafer marks 221X and 221Y obtained by then-th measuring operation (n=1 to 10), that is, the measured arraycoordinates of the center of the n-th sample shot, are assumed to be(Mxn,Myn). Next, the design array coordinates (Dxy,Dyn) of the wafermarks 221X and 221Y are substituted into the right-hand side of Eq. (1)to obtain computational array coordinate values (Fxn,Fyn). Then,deviations of the measured coordinate values (Mxn,Myn) from thecomputational array coordinate values (Fxn,Fyn), that is, alignmenterrors (Exn,Eyn)(=(Mxn-Fxn,Myn-Fyn)), are obtained. Thereafter, valuesof the six EGA parameters are determined so as to minimize the sum ofthe squares of the alignment errors obtained for all the sample shots,that is, the residual error component.

[0032] Assuming that the number of measured sample shots is K (K=10 inFIG. 39(a)), the residual error component is expressed by the followingequation (2). For example, values of the six EGA parameters (scalingparameters Rx, Ry, wafer rotation Θ, perpendicularity W, and offsetquantities Ox, Oy) are obtained by solving simultaneous equationsestablished by setting the result of partial differentiation of Eq. 2with respect to each of the six EGA parameters equal to zero.$\begin{matrix}{{{Residual}\quad {error}\quad {component}} = {\sum\limits_{n = 1}^{K}\left\{ {\left( {{Mxn} - {FXn}} \right)^{2} + \left( {{Myn} - {Fyn}} \right)^{2}} \right\}}} & (2)\end{matrix}$

[0033] Next, the six EGA parameter values thus obtained and the designarray coordinate values (Dxm,Dym) of each shot area SEm (m=1 to M) aresequentially substituted into the right-hand side of Eq. (1), therebyobtaining array coordinate values in the stage coordinate system of eachshot area SEm of the critical layer on the wafer 20. Assuming that thedemagnification ratio for the critical layer is 5:1, while thedemagnification ratio for the middle layer is 2.5:1, that is, theexposure field of the projection exposure apparatus for the middle layeris 2 times as large as the exposure field of the projection exposureapparatus for the critical layer in both the directions X and Y, eachmiddle layer shot area contains four critical layer shot areas.

[0034] Therefore, when exposure is to be carried out by the middle layerprojection exposure apparatus, the critical layer shot areas SEm (m=1 toM) shown in FIG. 39(a) are divided into a plurality of blocks eachcomprising two shot areas in the direction X and two shot areas in thedirection Y, and array coordinates in the stage coordinate system of thecenter of each block are obtained from the computational arraycoordinates of the four shot areas in the block. Thereafter, the arraycoordinates of the center of each block on the wafer 20 are sequentiallyaligned with the center of the exposure field of the middle layerprojection exposure apparatus, and a pattern image of a reticle for themiddle layer, which contains vernier marks, is transferred onto eachblock by exposure. After the exposure process, the wafer 20 is subjectedto development process.

[0035]FIG. 40(a) shows the wafer 20 having overlaid vernier marks formedby the middle layer projection exposure apparatus. In FIG. 40(a), shotareas SF1, SF2, . . . , SFN (N is an integer of 3 or more, for example)of the middle layer are arrayed on the wafer 20 at a predetermined pitchalong each of the X- and Y-axes, and each shot area SFn (n=1 to N)contains four critical layer shot areas. Further, the center 261 of eachshot area SFn is approximately coincident with the center of thecorresponding block of four critical layer shot areas. Each shot areaSFn has 20 (=4×5) vernier marks corresponding to a total of 20 verniermarks of the critical layer, that is, four groups of five vernier marks222A to 222E (see FIG. 40(b)).

[0036] Here, four shot areas SFa to SFd (shaded shot areas in FIG.40(a)) are defined as objects to be measured, and amounts of positionaldisplacement of the middle layer vernier marks relative to the criticallayer vernier marks are measured, for example, at measuring points 262to 265 selected at random in the shot areas SFa to SFd. FIG. 40(b) showsthe shot area SFa among the four. In FIG. 40(b), the middle layer shotarea SFa has middle layer vernier marks 224A to 224E, 226A to 226E, 228Ato 228E, and 230A to 230E formed to surround the vernier marks,respectively, which belong to four critical layer shot areas SEp,SE(p+1), SEq and SE(q+1), which underlie the shot area SFa. Accordingly,at the measuring point 262 in the shot area SFa, an amount of positionaldisplacement in the direction X or Y of the middle layer vernier mark226C relative to the critical layer vernier mark 222C in the shot areaSE(p+1) is measured. Similarly, an amount of positional displacementbetween the two corresponding vernier marks is measured at each of themeasuring points 263 to 265.

[0037] Consequently, if all the critical layer vernier marks aredisplaced, for example, by a predetermined amount δX in the direction Xrelative to the middle layer vernier marks at all the measuring points262 to 265, in FIG. 40(a), it is revealed that the X-axis offset Ox inthe EGA parameters has a residual error δX. Therefore, the residualerror is previously stored in a control system of the middle layerprojection exposure apparatus as a system constant to correct analignment result, thereby making it possible to form a middle layerpattern over the critical layer by exposure with high overlay accuracy.

[0038] Thus, residual errors of the EGA parameters can be corrected bymeasuring amounts of positional displacement between the critical layervernier marks and the middle layer vernier marks. However, no particularconsideration has heretofore been given to the arrangement of measuringpoints for measuring amounts of positional displacement between thecritical layer vernier marks and the middle layer vernier marks, asshown by the measuring points 262 to 265 in FIG. 40(a). Accordingly,when the projected image for the middle layer has a magnification erroror a rotation error, for example, the magnification or rotation errormay be erroneously judged to be a residual error of the EGA parameters.

[0039] The above problem will be explained below with reference to FIGS.40(a) to 41(b). FIG. 41(a) shows a state where the middle layer shotarea SFa is slightly enlarged relative to a projected image 266 obtainedwhen there is no magnification error. As shown in FIG. 41(a), in thecentral portion at the right end of the first quadrant of the shot areaSFa (i.e. the critical layer shot area SE(p+1)), the middle layervernier mark 226C is displaced relative to the critical layer verniermark 222C by Δx1 and Δy1 in the directions X and Y, respectively. In thecenter portion at the right end of the second quadrant (i.e. the shotarea SEp), the middle layer vernier mark 224C is displaced relative tothe critical layer vernier mark 222C by approximately Δy 1 in thedirection Y, but the amount of displacement in the direction X of themiddle layer vernier mark 224C is so small as to be ignorable.Similarly, in the third quadrant (i.e. the shot area SEq) and the fourthquadrant (i.e. the shot area SE(q+1)), the two vernier marks aredisplaced in symmetric relation to those in the second and firstquadrants, respectively.

[0040] When a projected image of the middle layer has such amagnification error, if an amount of positional displacement in thedirection X between the two corresponding vernier marks is measured atthe measuring point 265 in the first quadrant of the shot area SFd,shown in FIG. 40(a), and at the measuring point 263 in the secondquadrant of the shot area SFb, shown in FIG. 40(a), the results of themeasurement are Δx 1 and 0, respectively. Accordingly, if residualerrors of the EGA parameters of Eq. (1) are obtained by simplyprocessing these amounts of positional displacement, predeterminederrors remain in the scaling parameter Rx and offset Ox in the directionX, respectively.

[0041] If an amount of positional displacement in the direction Xbetween the two corresponding vernier marks is measured at the measuringpoint 262 in the first quadrant of the shot area SFa, shown in FIG.40(a), and at the measuring point 264 in the second quadrant of the shotarea SFc, shown in FIG. 40(a), the results of the measurement are Δx 1and 0, respectively. Accordingly, if residual errors of the EGAparameters of Eq. (1) are obtained by simply processing these amounts ofpositional displacement, predetermined errors remain in theperpendicularity W and the offset Ox in the direction X, respectively.That is, if an amount of positional displacement in the direction Xbetween two corresponding vernier marks is measured at measuring pointsin middle layer shot areas defined as objects to be measured, whichmeasuring points are in different columns on the critical layer, themagnification error of the middle layer may be mistaken for a residualerror (linear error) in the EGA parameters. Such erroneous recognitionmay also occur in the case of measuring an amount of positionaldisplacement in the direction Y between two corresponding vernier marks.

[0042]FIG. 41(b) shows a state where the middle layer shot area SFa hasbeen rotated counterclockwise relative to the projected image 266obtained when there is no error (i.e. a state where the shot area SFahas a shot rotation error). As shown in FIG. 41(b), in the centralportion at the right end of the first quadrant of the shot area SFa, themiddle layer vernier mark 226C is displaced relative to the criticallayer vernier mark 222C by −x2 and Δy2 in the directions X and Y,respectively. In the central portion at the right end of the secondquadrant (i.e. the shot area SEp), the middle layer vernier mark 224C isdisplaced relative to the critical layer vernier mark 222C byapproximately −Δx3 in the direction X, but the amount of displacement inthe direction Y of the middle layer vernier mark 224C is so small as tobe ignorable. Similarly, in the third and fourth quadrants, the twocorresponding vernier marks are displaced in symmetric relation to thosein the second and first quadrants, respectively.

[0043] When a projected image of the middle layer has such a rotationerror, if an amount of positional displacement in the direction Xbetween two corresponding vernier marks is measured at the measuringpoint 265 in the first quadrant of the shot area SFd, shown in FIG.40(a), and at the measuring point 263 in the second quadrant of the shotarea SFb, shown in FIG. 40(a), the results of the measurement are −Δx2and −Δx3, respectively. Accordingly, if residual errors in the EGAparameters of Eq. (1) are obtained by simply processing these amounts ofpositional displacement, an error remains in a parameter other than theoffset Ox among the EGA parameters of Eq. (1). When an amount ofpositional displacement in the direction Y between two correspondingvernier marks is measured at each of the measuring points 265 and 263,an error similarly remains in an EGA parameter other than the offset Oy.Thus, it will be understood that, when amounts of positionaldisplacement between the critical layer vernier marks and the middlelayer vernier marks are measured to correct residual errors of the EGAparameters, a mere magnification error or rotation error of a middlelayer shot area may be mistaken for a residual error of an EGA parameterother than the offsets Ox and Oy depending upon the selection of thepositions of measuring points in middle layer shot areas as objects tobe measured.

[0044] When critical layer shot areas (chip patterns) have amagnification error or a rotation error (chip rotation), such an errormay also be mistaken for a residual error of an EGA parameter other thanthe offsets Ox and Oy depending upon the selection of measuring pointsfor measuring amounts of positional displacement between thecorresponding vernier marks.

[0045] As has been described above, residual errors of the EGAparameters can be corrected by measuring amounts of positionaldisplacement between the critical layer vernier marks and the middlelayer vernier marks. However, there may be residual errors not only inthe above-described coordinate transformation parameters related to thewhole wafer but also in so-called in-shot parameters comprising shotmagnifications (i.e. linear expansion and contraction of each chippattern in the directions X and Y) rx and ry, shot rotation (i.e. arotation angle of each chip pattern) θ, and shot perpendicularity (i.e.a perpendicularity error of the coordinate system in each chip pattern)w.

[0046] To obtain a correction value for the shot magnification rx, forexample, it is conceivable to measure an amount of positionaldisplacement between two corresponding vernier marks at each of the twoopposite measuring points 262 and 266 in the shot area SFa shown in FIG.40(a). A residual shot magnification error, i.e. a correction value forthe shot magnification rx, should be calculable from the differencebetween the X components of the amounts of positional displacementmeasured at the two measuring points 262 and 266. Similarly, a residualshot rotation error should be calculable.

[0047] In actual practice, however, the vernier mark positions on thecritical layer may have different stepping errors because the measuringpoints 262 and 266 belong to different shot areas SE(p+1) and SEp on thecritical layer. That is, if no consideration is given to the arrangementof measuring points at which vernier marks are to be read, variation dueto the stepping accuracy of the wafer stage may be mistaken for aresidual shot magnification error or a residual shot rotation error. Ifan erroneous correction value is used to correct the correspondingin-shot parameter, the alignment accuracy reduces disadvantageously.

SUMMARY OF THE INVENTION

[0048] In view of the above-described problems, an object of the presentinvention is to provide an exposure method capable of minimizing anoverlay error when a preceding layer has a perpendicularity error in anarray of shot areas or a shot rotation in a case where exposure iscarried out by the mix-and-match method using a plurality of exposureapparatuses having respective exposure fields of different sizes.

[0049] Another object of the present invention is to provide an exposuremethod capable of minimizing an overlay error when a perpendicularityerror remains in a shot array on a first layer in a case where exposureis carried out by the mix-and-match method using a plurality of exposureapparatuses which are different from each other in the size of exposurefield (shot area) on a photosensitive substrate.

[0050] Still another object of the present invention is to provide anexposure method capable of increasing an overlay accuracy between acritical layer pattern and a middle layer pattern in a case whereexposure is carried out by the mix-and-match method with respect to asubstrate where a critical layer and a middle layer are mixedly present.

[0051] The present invention provides an exposure method in which maskpatterns are overlaid on one another on a substrate, which is an objectto be exposed, by using a first and second exposure apparatuses havingrespective exposure fields of different sizes. The exposure methodincludes the steps of: sequentially transferring a first mask patternonto the substrate in the form of a first array in units of a shot areaof a predetermined size by using the first exposure apparatus; detectingat least either one of a perpendicularity error of the first array froma design value and a mean value of rotation angles of the shot areas inthe first array when a second mask pattern is to be sequentiallytransferred onto the substrate in the form of a second array in units ofa shot area different in size from the unit shot area of a predeterminedsize by using the second exposure apparatus; and rotating the secondmask pattern and the substrate relative to each other through an anglecorresponding to a result of the detection, and thereafter, sequentiallytransferring the second mask pattern onto the substrate.

[0052] The function of the above-described exposure method according tothe present invention will be explained below. Let us assume that theperpendicularity of the first array, which is an array of shot areas towhich the first mask pattern is to be transferred, has an angle W ofdeviation from a design value (90° in general). Further, it is assumedthat each of these shot areas has a square outer shape, and that oneshot area in a second shot array to which a second mask pattern is to betransferred is laid over four shot areas in the first array. In thiscase, according to the present invention, the second mask pattern andthe substrate are rotated relative to each other so that the angle δ ofrotation of the shot area in the second array, which is defined aboutthe center of the four shot areas, is W/4. By doing so, the overlayerror between the first mask pattern image and the second mask patternimage on the substrate is reduced to a minimum on the average.

[0053] On the other hand, when the perpendicularity error of the firstarray is zero and the shot rotation of the four shot areas is W, therotation angle δ of the shot area over the four shot areas is also setat W/4, whereby the overlay error between the first mask pattern imageand the second mask pattern image on the substrate is reduced to aminimum on the average.

[0054] In addition, the present invention provides another exposuremethod in which mask patterns are overlaid on one another on asubstrate, which is an object to be exposed, by using a first exposureapparatus having a first exposure field of a predetermined size, and asecond exposure apparatus which scans a mask and the substratesynchronously to sequentially transfer a pattern formed on the mask ontothe substrate, and which has a second exposure field different in sizefrom the first exposure field. The exposure method includes the stepsof: sequentially transferring a first mask pattern onto the substrate inthe form of a first array in units of a shot area of a predeterminedsize by using the first exposure apparatus; detecting at least eitherone of a perpendicularity error of the first array from a design valueand a mean value of rotation angles of the shot areas in the first arraywhen a second mask pattern is to be sequentially transferred onto thesubstrate in the form of a second array over the first array in units ofa shot area different in size from the unit shot area of a predeterminedsize by using the second exposure apparatus; and displacing the secondmask pattern and the substrate relative to each other in a directionperpendicular to a scanning direction of the second exposure apparatusby a distance corresponding to a result of the detection, andthereafter, sequentially transferring the second mask pattern onto thesubstrate by a scanning exposure method.

[0055] In this case, it is desirable to rotate the second mask patternand the substrate relative to each other through an angle correspondingto the result of detection of at least either one of a perpendicularityerror of the first array from a design value and a mean value ofrotation angles of the shot areas in the first array.

[0056] In the above-described exposure method according to the presentinvention, a scanning type exposure apparatus such as a step-and-scanexposure apparatus is used as the second exposure apparatus. Let usassume that the width in one direction (e.g. a direction Y) of each shotarea in the first array is L, and that the first array has aperpendicularity error W. Further, the width in the direction Y of eachshot area in the second array formed by the second exposure apparatus isassumed to be (3/2)L. In this case, if exposure is carried out with thecenters of shot areas in the second array being merely aligned with thecenter line of the first array, an overlay error of a predeterminedmaximum width occurs between the first and second mask pattern images.

[0057] Therefore, in the above-described method according to the presentinvention, exposure is carried out with the centers of shot areas in thesecond array being displaced relative to the center line of the firstarray in a direction perpendicular to the direction Y by a width d(≈L·W/4). By doing so, the overlay error between the first mask patternimage and the second mask pattern image on the substrate is reduced to aminimum on the average. In a case where the first array has a shotrotation also, the overlay error can be minimized by displacing theposition of each shot area in the second array.

[0058] Further, if the rotation that is used in the first exposuremethod is used in the second exposure method, the overlay error isfurther reduced.

[0059] In addition, the present invention provides another exposuremethod in which a first mask pattern is transferred onto aphotosensitive substrate in the form of a predetermined array by using afirst exposure apparatus having a first exposure field of apredetermined shape, and a second mask pattern is transferred onto thephotosensitive substrate over the first mask pattern array by using asecond exposure apparatus having a second exposure field different fromthe first exposure field in length in a predetermined direction. In theexposure method, when the first mask pattern is to be transferred ontothe photosensitive substrate by using the first exposure apparatus, anarray of a plurality of shot areas to each of which the first maskpattern is to be transferred is set on the photosensitive substratealong a direction (X1) corresponding to the direction in which the firstexposure field is different in length from the second exposure field.

[0060] According to the above-described exposure method of the presentinvention, the array of a plurality of shot areas to which the firstmask pattern is to be transferred is such that an imaginary straightline passing through shot areas which are adjacent to each other in thedirection X1, which corresponds to the direction in which the firstexposure field is different in length from the second exposure field, isparallel to the direction X1. As a result, when shot areas of a secondlayer are arrayed over the shot areas of the first layer by using thesecond exposure apparatus, the overlay error is minimized even if theshot array of the first layer has a perpendicularity error.

[0061] In this exposure method, when the first mask pattern is to betransferred onto the photosensitive substrate by using the firstexposure apparatus, the photosensitive substrate and the first maskpattern have previously been rotated through 90° from their ordinarypositions. By doing so, even if a perpendicularity error is present inthe shot array of the first layer, the shot areas of the first layer canbe arrayed in a straight-line form along the direction (X1)corresponding to the direction in which the first exposure field isdifferent in length from the second exposure field.

[0062] One example of the second exposure apparatus is a scanningexposure type exposure apparatus; in this case, it is desirable that theabove-described predetermined direction should be the scanningdirection. The reason for this is that the exposure field of a scanningexposure type exposure apparatus can be readily lengthened in thescanning direction.

[0063] In addition, the present invention provides another exposuremethod in which mask patterns are overlaid on one another on aphotosensitive substrate, which is an object to be exposed, by using afirst exposure apparatus having a first exposure field of apredetermined size on the photosensitive substrate, and a secondexposure apparatus having a second exposure field which is M₁/N₁ times(M₁ and N₁ are integers; M₁>N₁) as large as the first exposure field ina first direction and which is M₂/N₂ times (M₂ and N₂ are integers;M₂≧N₂) as large as the first exposure field in a second direction whichis perpendicular to the first direction. The exposure method has thefirst step of sequentially transferring an image of a first maskpattern, which has an alignment mark and a first overlay accuracymeasuring mark, onto the photosensitive substrate in the form of atwo-dimensional array extending in the first and second directions inunits of the first exposure field by using the first exposure apparatus.

[0064] The exposure method according to the present invention furtherhas: the second step of transferring an image of a second mask pattern,which has a second overlay accuracy measuring mark, over a plurality ofimages of the first mask pattern, which have been transferred onto thephotosensitive substrate in the first step, in a two-dimensional arrayextending in the first and second directions on the photosensitivesubstrate in units of the second exposure field with reference to theposition of the image of the alignment mark by using the second exposureapparatus; and the third step of dividing an exposure area on thephotosensitive substrate into a plurality of reference measurement areasin units of an area which is N₁ times as large as the width of thesecond exposure field in the first direction on the photosensitivesubstrate and which is N₂ times as large as the width of the secondexposure field in the second direction on the photosensitive substrate,and measuring an amount of positional displacement between the images ofthe first and second overlay accuracy measuring marks lying at themutually identical positions in a predetermined number of referencemeasurement areas selected from among the plurality of referencemeasurement areas, thereby obtaining a correction value which is usedwhen the position of the alignment mark image transferred by the firstexposure apparatus is detected by the second exposure apparatus on thebasis of the amount of positional displacement measured as describedabove. Thereafter, the exposure position is corrected by using thecorrection value obtained in the third step when overlay exposure iscarried out by using the second exposure apparatus with respect to thesurface of the photosensitive substrate exposed by the first exposureapparatus.

[0065] In this case, it is desirable for the second exposure apparatusto calculate the exposure position on the basis of the alignment markimage and by use of a predetermined coordinate transformation parameter(EGA parameter) and to obtain a correction value for the coordinatetransformation parameter in the third step.

[0066] According to the above-described exposure method of the presentinvention, the first exposure apparatus is used, for example, forexposure of a critical layer, and the second exposure apparatus is used,for example, for exposure of a middle layer because the second exposurefield is larger than the first exposure field. Further, because thesecond exposure field is M₁/N₁ times and M₂/N₂ times as large as thefirst exposure field in the first and second directions, respectively,if the widths in the first and second directions of the first maskpattern image formed by the first exposure apparatus are denoted by dand c, respectively, the widths in the first and second directions ofthe second mask pattern image formed by the second exposure apparatusare dM₁/N₁ and cM₂/N₂, respectively.

[0067] Assuming that the integers M₁ and N₁ have no common divider otherthan 1, and the integers M₂ and N₂ also have no common divider otherthan 1, an area on the photosensitive substrate which has a sizeregarded as being the least common multiple of the sizes of the firstand second mask pattern images is an area which has a width dM₁ in thefirst direction and a width cM₂ in the second direction, that is, areference measurement area which is N₁ times and N₂ times as large asthe width of the second exposure field in the first and seconddirections, respectively. Such a reference measurement area contains aninteger number of first and second mask pattern images in each of thefirst and second directions.

[0068] If M₁=2, N₁=1, M₂=2, and N₂=1, for example, the second maskpattern image itself is the reference measurement area. In such a case,in the present invention, if an amount of positional displacementbetween two corresponding alignment mark images is measured at ameasuring point in the top right portion of the first referencemeasurement area, an amount of positional displacement between twocorresponding alignment mark images is similarly measured at a measuringpoint in the top right portion of each of the second to fourth referencemeasurement areas. Thus, a magnification error or rotation error of thesecond exposure field is approximately equally introduced into all themeasured amounts of positional displacement. Accordingly, there is nolikelihood that a magnification error or rotation error of the secondexposure field will be mistaken for an error component other than anoffset component in the amount of positional displacement between thefirst and second mask pattern images. Thus, the overlay accuracyimproves.

[0069] In this case, if an alignment method in which coordinatetransformation parameters are employed, e.g. the EGA method, is used forthe exposure process carried out by the second exposure apparatus, thereis no likelihood that a magnification error or rotation error of thesecond exposure field will be mistaken for a coordinate transformationparameter other than an offset.

[0070] In addition, the present invention provides another exposuremethod in which mask patterns are overlaid on one another on aphotosensitive substrate by using a first and second exposureapparatuses having respective exposure fields of different sizes. In theexposure method, images of a first and second mask patterns containingoverlay accuracy measuring marks are sequentially transferred onto aphotosensitive substrate for evaluation, being overlaid on one another,by using the first and second exposure apparatuses, and an amount ofpositional displacement between the overlaid images of the overlayaccuracy measuring marks is measured at a predetermined measuring pointin a reference measurement area on the evaluation photosensitivesubstrate in which a shot area formed in units of the exposure field ofthe first exposure apparatus and a shot area formed in units of theexposure field of the second exposure apparatus are overlaid on oneanother such that neither of the overlaid shot areas extends over beyonda part of the reference measurement area (or neither of the overlaidshot areas extends over a plurality of shot areas). On the basis of theresult of the measurement, alignment or correction of image-formationcharacteristics is effected when exposure is to be carried out by thesecond exposure apparatus with respect to the surface of thephotosensitive substrate exposed by the first exposure apparatus.

[0071] In the above-described exposure method according to the presentinvention, the reference measurement area does not extend over aplurality of shot areas in either of two layers. Therefore, the amountof positional displacement measured at a measuring point in thereference measurement area contains no effect of stepping error ofeither of the exposure apparatuses. Accordingly, a high overlay accuracyis obtained by carrying out exposure after a parameter for alignment ora parameter indicating image-formation characteristics has beencorrected on the basis of the measured amount of positionaldisplacement.

[0072] In addition, the present invention provides another exposuremethod in which mask patterns are overlaid on one another on aphotosensitive substrate, which is an object to be exposed, by using afirst exposure apparatus having a first exposure field of apredetermined size on the photosensitive substrate, and a secondexposure apparatus having a second exposure field which is M₁/N₁ times(M₁ and N₁ are integers; M₁≠N₁) as large as the first exposure field ina first direction and which is M₂/N₂ times (M₂ and N₂ are integers) aslarge as the first exposure field in a second direction which isperpendicular to the first direction. The exposure method has: the firststep of sequentially transferring an image of a first mask pattern,which has an alignment mark and a first overlay accuracy measuring mark,onto a plurality of first shot areas arrayed on the photosensitivesubstrate in units of the first exposure field by using the firstexposure apparatus; and the second step of sequentially transferring animage of a second mask pattern, which has a second overlay accuracymeasuring mark, onto a plurality of second shot areas arrayed on thephotosensitive substrate, exposed in the first step, in units of thesecond exposure field with reference to the image of the alignment markby using the second exposure apparatus.

[0073] Further, the exposure method according to the present inventionhas the third step of defining a plurality of reference measurementareas on the photosensitive substrate in each of which any one of thefirst shot areas and any one of the second shot areas are overlaid onone another such that neither of the overlaid shot areas extends overbeyond a part of the reference measurement area, and measuring an amountof positional displacement between the images of the first and secondoverlay accuracy measuring marks lying at the mutually identicalpositions in a predetermined number of reference measurement areasselected from among the plurality of reference measurement areas,thereby obtaining a correction value which is used when the position ofthe alignment mark image transferred by the first exposure apparatus isdetected by the second exposure apparatus on the basis of the amount ofpositional displacement measured as described above.

[0074] In this case, the first exposure apparatus is used, for example,for a critical layer, and the second exposure apparatus is used, forexample, for a middle layer. In the exposure method, if a plurality ofmeasuring points for measuring an amount of positional displacementbetween a pair of overlay accuracy measuring marks are set in eachreference measurement area in order to obtain a linear expansion andcontraction error or the like in a shot area, the distribution of themeasuring points does not extend over a plurality of first shot areasnor a plurality of second shot areas. Accordingly, the linear expansionand contraction error or the like can be accurately obtained withoutbeing affected by stepping errors of the critical and middle layers, andthe overlay accuracy between the critical and middle layers is improvedby correcting the linear expansion and contraction error at the middlelayer.

[0075] In this case, each alignment mark and each first overlay accuracymeasuring mark may be the same mark.

[0076] One example of the correction value obtained in the third step isa correction value for a parameter indicating a predeterminedimage-formation characteristic calculated on the basis of the positionsof the alignment mark images. One example of such a parameter is atleast one parameter selected from a parameter group consisting of shotmagnifications rx and ry, shot rotation θ, and shot perpendicularity w.In this case, it is desirable to correct the image-formationcharacteristic by using the correction value obtained in the third stepwhen overlay exposure is to be carried out thereafter by using thesecond exposure apparatus with respect to the surface of thephotosensitive substrate exposed by the first exposure apparatus.

[0077] Further, one example of the first exposure apparatus is aone-shot exposure type projection exposure apparatus, and one example ofthe second exposure apparatus is a scanning exposure type projectionexposure apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0078]FIG. 1 is a perspective view schematically showing an exposuresystem used in a first example of a first embodiment of the exposuremethod according to the present invention.

[0079]FIG. 2(a) illustrates the detection principle of a laser stepalignment type alignment system.

[0080]FIG. 2(b) is an enlarged view showing one example of a wafer markwhich is used in another type of alignment system.

[0081]FIG. 3 is a plan view showing a shot array of a critical layer ona wafer in the first example.

[0082]FIG. 4 is an enlarged plan view showing a part of the shot arrayshown in FIG. 3, together with a shot area of a middle layer exposedover the shot array.

[0083]FIG. 5 is a plan view showing a shot array of a middle layerexposed over the shot array shown in FIG. 3 in the first example.

[0084]FIG. 6(a) is a plan view of a second example of the firstembodiment of the present invention, showing an array of shot areasexposed on a first layer, together with one example of a shot array in acase where shot areas exposed over the first-layer shot areas have alarge overlay error.

[0085]FIG. 6(b) is a plan view showing a shot area exposed by a scanningtype second exposure apparatus.

[0086]FIG. 7 is a plan view showing an array of shot areas in which theoverlay error reduces in the second example of the first embodiment.

[0087] FIGS. 8(a), 8(b) and 8(c) illustrate an alignment method foroverlay exposure in which short shot areas are overlaid on an array oflong shot areas in the second example of the first embodiment.

[0088]FIG. 9(a) is a plan view of a third example of the firstembodiment of the present invention, showing a shot array on a wafer.

[0089]FIG. 9(b) is a plan view showing a shot area exposed by a secondexposure apparatus.

[0090]FIG. 10(a) is a plan view showing a part of the shot array shownin FIG. 9(a), together with one example of an array in a case where shotareas exposed over the shot array have a large overlay error.

[0091]FIG. 10(b) is a plan view showing an array of shot areas in whichthe overlay error reduces.

[0092]FIG. 10(c) illustrates an alignment method for overlay exposure inwhich short shot areas are overlaid on an array of long shot areas.

[0093]FIG. 11 is a perspective view schematically showing an exposuresystem used in a first example of a second embodiment of the exposuremethod according to the present invention.

[0094]FIG. 12(a) is a plan view showing the orientation of a reticlewhen exposure is carried out for a first layer on a wafer in the firstexample of the second embodiment.

[0095]FIG. 12(b) is a plan view showing the orientation of a wafer whenexposure is carried out for the first layer on the wafer.

[0096]FIG. 13 is a plan view for explaining an alignment method executedbefore exposure is carried out for the second layer on the wafer in thefirst example of the second embodiment.

[0097]FIG. 14(a) is a plan view showing a shot array when exposure iscarried out for a second layer on a wafer in the first example of thesecond embodiment.

[0098]FIG. 14(b) is a plan view showing the orientation of a reticlewhen exposure is carried out for the second layer.

[0099]FIG. 14(c) is a plan view showing first-layer shot areas on thewafer.

[0100]FIG. 15 is a plan view illustrating a second example of the secondembodiment, showing a wafer which is placed on a wafer stage of ascanning type exposure apparatus after it has been subjected to exposurefor a first layer.

[0101]FIG. 16 is a plan view showing a shot array of a second layer onthe wafer in the second example of the second embodiment.

[0102]FIG. 17(a) is a plan view of a third example of the secondembodiment, showing shot arrays of first and second layers on a wafer.

[0103]FIG. 17(b) is a plan view showing the orientation of a reticlewhen exposure is carried out for the second layer on the wafer.

[0104]FIG. 17(c) is a plan view showing the orientation of a reticlewhen exposure is carried out for the first layer on the wafer.

[0105]FIG. 18 is a perspective view schematically showing an exposuresystem used in a third embodiment of the exposure method according tothe present invention.

[0106]FIG. 19(a) is a plan view showing a shot array of a critical layeron a wafer in the third embodiment.

[0107]FIG. 19(b) is an enlarged plan view showing an arrangement ofvernier marks in a shot area of the critical layer.

[0108]FIG. 20(a) is a plan view showing a shot array and measuring pointarrangement on a middle layer exposed over the critical layer shown inFIG. 19(a).

[0109]FIG. 20(b) is an enlarged plan view showing a part of the verniermark arrangement in a shot area of the middle layer.

[0110] FIGS. 21(a) and 21(b) show another example of the arrangement ofmeasuring points on the wafer.

[0111]FIG. 22(a) is an enlarged view showing another example of thearrangement of measuring points in a shot area of the middle layer.

[0112]FIG. 22(b) is an enlarged view showing still another example ofthe arrangement of measuring points in a shot area of the middle layer.

[0113] FIGS. 23(a) and 23(b) are plan views each showing one example ofa desirable arrangement of measuring points on a wafer.

[0114] FIGS. 24(a) and 24(b) show one example of an arrangement ofmeasuring points in a case where measuring points are selected fromthose which are at different positions in a plurality of referencemeasurement areas on a wafer.

[0115] FIGS. 25(a), 25(b) and 25(c) show an example of referencemeasurement areas used in a case where a plurality of chip patterns fitin each shot area of a critical layer.

[0116] FIGS. 26(a), 26(b) and 26(c) show an example of referencemeasurement areas used in a case where shot areas of a middle layer areexposed by a scanning exposure method.

[0117] FIGS. 27(a) and 27(b) show that the expansion and contractionquantity of shot areas of a middle layer differs according to measuringpoints in the example shown in FIGS. 26(a) to 26(c).

[0118] FIGS. 28(a), 28(b) and 28(c) show an example of referencemeasurement areas used in a case where shot areas of a middle layer areexposed by a scanning exposure method, and shot areas of a criticallayer are wider than the middle layer shot areas.

[0119]FIG. 29 is a perspective view schematically showing an exposuresystem used in a fourth embodiment of the exposure method according tothe present invention.

[0120]FIG. 30(a) is a plan view showing a shot array of a critical layeron a wafer in the fourth embodiment of the present invention.

[0121]FIG. 30(b) is an enlarged plan view showing an arrangement ofvernier marks in a shot area of the critical layer shown in FIG. 30(a).

[0122]FIG. 31(a) is a plan view showing a shot array of a middle layerexposed over the critical layer shown in FIG. 30(a).

[0123]FIG. 31(b) is an enlarged plan view showing vernier markarrangements on two layers in a shot area of the middle layer shown inFIG. 31(a).

[0124]FIG. 32 is an enlarged plan view showing one example of referencemeasurement areas and measuring points set on the wafer shown in FIG.31(a).

[0125] FIGS. 33(a), 33(b) and 33(c) show the way in which referencemeasurement areas are determined in a case where a middle layer shotarea is twice as large as a critical layer shot area in each ofdirections X and Y.

[0126] FIGS. 34(a), 34(b) and 34(c) show the way in which referencemeasurement areas are determined in a case where first-layer shot areasare exposed by a one-shot exposure method, while second-layer shot areasare exposed by a scanning exposure method, and the first-layer shotareas are wider than the second-layer shot areas.

[0127]FIG. 35 is a view for explanation of a background art related tothe present invention, showing an overlay error due to aperpendicularity error of the shot array on the preceding layer.

[0128]FIG. 36 is a view for explanation of a background art related tothe present invention, showing an overlay error due to a shot rotationof the shot array on the preceding layer.

[0129] FIGS. 37(a), 37(b) and 37(c) are views for explanation of abackground art related to the present invention, showing one example ofa mix-and-match exposure process.

[0130] FIGS. 38(a), 38(b) and 38(c) are views for explanation of abackground art related to the present invention, showing a case where anoverlay error arises in a mix-and-match exposure process.

[0131]FIG. 39(a) is a plan view of a background art related to thepresent invention, showing a shot array of a critical layer on a wafer.

[0132]FIG. 39(b) is an enlarged plan view showing an arrangement ofvernier marks in a shot area of the critical layer shown in FIG. 39(a).

[0133]FIG. 40(a) is a plan view of a shot array and measuring pointarrangement on a middle layer exposed over the critical layer shown inFIG. 39(a).

[0134]FIG. 40(b) is an enlarged plan view showing an arrangement ofvernier marks in a shot area of the middle layer shown in FIG. 40(a).

[0135] FIGS. 41(a) and 41(b) illustrate a background art related to thepresent invention, in which FIG. 41(a) shows a shot area of a middlelayer which has a magnification error, and FIG. 41(b) shows a middlelayer shot area which has a rotation error.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0136] A first example of a first embodiment of the exposure methodaccording to the present invention will be described below withreference to FIGS. 1 to 5. In this embodiment, two exposure apparatusesare used: a first exposure apparatus of the stepper type (one-shotexposure type) with a demagnification ratio of 5:1, and a secondexposure apparatus of the stepper type with a demagnification ratio of2.5:1. In this case, one shot area exposed by the second exposureapparatus corresponds to four shot areas exposed by the first exposureapparatus.

[0137]FIG. 1 shows an exposure system used in an exposure methodaccording to the first embodiment of the present invention. In theexposure system shown in FIG. 1 are installed a first exposure apparatus1A of the stepper type which has a small exposure field, and a secondexposure apparatus 1B of the stepper type which has a large exposurefield. In this embodiment, the exposure apparatus 1A is ahigh-resolution exposure apparatus, while the exposure apparatus 1B is alow-resolution exposure apparatus. The high-resolution exposureapparatus 1A is used to carry out exposure for a critical layer on awafer, and the low-resolution exposure apparatus 1B is used to carry outexposure for a middle layer on the wafer. However, the exposureapparatus 1A may be a low-resolution exposure apparatus or the exposureapparatus 1B may be a high-resolution exposure apparatus according tothe kind of semiconductor device to be produced.

[0138] First, in the exposure apparatus 1A, a pattern area 2A on areticle RA is illuminated by exposure light from an illumination opticalsystem (not shown), and an image of a pattern formed in the pattern area2A is formed on an exposure field 4A on a wafer 20 as a projected imagereduced to ⅕ by a projection optical system 3A. A Z1-axis is taken in adirection parallel to an optical axis of the projection optical system3A, and two axes of an orthogonal coordinate system set in a planeperpendicular to the Z1-axis are defined as an X1-axis and a Y1-axis,respectively. The reticle RA has an alignment mark 17X for the X1-axisformed at an end of the pattern area 2A in the direction Y1 (e.g. withina masking frame) and also has an alignment mark 17Y for the Y1-axisformed at an end of the pattern area 2A in the direction X1.

[0139] The wafer 20 is held on a wafer stage 5A. The wafer stage 5Acomprises a Z-stage for moving the wafer 20 in the direction Z1 to setan exposure surface of the wafer 20 which is to be exposed at the bestfocus position, and an XY-stage for positioning the wafer 20 in both thedirections of the X1- and Y1-axes. A pair of moving mirrors 6A and 8Awhich are perpendicular to each other are fixed on the wafer stage 5A.The coordinate in the direction X1 of the wafer stage 5A is measured bya combination of the moving mirror 6A and a laser interferometer 7Awhich is installed outside the wafer stage 5A. The coordinate in thedirection Y1 of the wafer stage 5A is measured by a combination of themoving mirror 8A and a laser interferometer 9A which is installedoutside the wafer stage 5A. The coordinates measured by the laserinterferometers 7A and 9A are supplied to a controller 10A whichcontrols operations of the whole apparatus. The controller 10A drivesthe wafer stage 5A to step in both the directions X1 and Y1 throughdrive units (not shown), thereby positioning the wafer 20. In this case,the stepping drive of the wafer 20 is effected according to an array ofshot areas (i.e. unit areas to each of which a pattern image of thepattern area 2A is to be projected by exposure) set on the exposuresurface of the wafer 20, that is, a shot map for a critical layer. Theshot map is generated by a map generating unit which comprises acomputer in the controller 10A.

[0140] The exposure apparatus 1A is provided with alignment systems 11Aand 14A both of which are TTL (Through-The-Lens) and laser stepalignment type (hereinafter referred to as “LSA type”) systems. An LSAtype alignment system is disclosed in detail, for example, in JP(A) No.60-130742. Therefore, only an outline of the alignment systems 11A and14A will be given below. A laser beam emitted from the alignment system11A for the X1-axis is reflected by a mirror 12A, which is disposedbetween the projection optical system 3A and the reticle RA, and thereflected laser beam enters the projection optical system 3A. The laserbeam emanating from the projection optical system 3A is converged ontoan area near the exposure field 4A in the form of a slit-shaped lightspot 13A elongated in the direction Y1.

[0141]FIG. 2(a) shows a wafer mark MX for the X1-axis which serves as analignment mark on the wafer 20, which is an object to be exposed. InFIG. 2(a), the wafer mark MX is a dot train pattern comprising recessesand projections, which are arranged at a predetermined pitch in adirection approximately parallel to the slit-shaped light spot 13A. Whenthe wafer mark MX is scanned in the direction X1 relative to theslit-shaped light spot 13A by driving the wafer stage 5A, shown in FIG.1, diffracted light is emitted in a predetermined direction as the wafermark MX coincides with the slit-shaped light spot 13A.

[0142] Referring to FIG. 1, the diffracted light returns to thealignment system 11A via the projection optical system 3A and the mirror12A. In the alignment system 11A, the diffracted light isphotoelectrically converted by a light-receiving element to obtain analignment signal. The alignment signal is supplied to the controller10A. In the controller 10A, the X1 coordinate of the wafer stage 5Ameasured when the alignment signal reaches a maximum, for example, issampled, thereby detecting the position of the wafer mark MX in thedirection of the X1-axis.

[0143] Similarly, a laser beam emitted from the LSA type alignmentsystem 14A for the Y1-axis enters the projection optical system 3A via amirror 15A and is converged onto the wafer 20 in the form of aslit-shaped light spot 16A elongated in the direction of the X1-axis.Diffracted light generated from the slit-shaped light spot 16A returnsto the alignment system 14A via the projection optical system 3A and themirror 15A. The alignment system 14A supplies an alignment signal to thecontroller 10A. Thus, the position in the Y1-axis direction of a wafermark for the Y1-axis on the wafer 20 is detected on the basis of thealignment signal.

[0144] It should be noted that, as each of the alignment systems 11A and14A, it is also possible to use a TTL (Through-The-Lens) type alignmentsystem or an off-axis type alignment system which detects the positionof a wafer mark without passing a wafer mark detecting light beamthrough the projection optical system 3A. As a wafer mark detectingmethod, it is also possible to use an image processing type detectionmethod, or a so-called two-beam interference type detection method inwhich two light beams are applied to a diffraction grating-shaped wafermark, and the position of the wafer mark is detected from a signalobtained from interference between a pair of diffracted light beamsgenerated in parallel from the illuminated wafer mark. When such animage processing type or two-beam interference type alignment system isused, a line-and-space pattern 22X as shown in FIG. 2(b) is used. Theline-and-space pattern 22X comprises recesses and projections, which arearranged at a predetermined pitch in the measuring direction, forexample.

[0145] Next, the second exposure apparatus 1B will be explained. Theexposure apparatus 1B has an arrangement approximately similar to thatof the above-described first exposure apparatus 1A. An image of apattern formed in a pattern area 2B of a reticle RB is projected througha projection optical system 3B onto an exposure field 4B on a wafer 20held on a wafer stage 5B as an image reduced to 1/2.5. A Z2-axis istaken in a direction parallel to an optical axis of the projectionoptical system 3B, and two axes of an orthogonal coordinate system setin a plane perpendicular to the Z2-axis are defined as an X2-axis and aY2-axis, respectively. The reticle RB has the pattern area divided intotwo columns in the direction X2 and two rows in the direction Y2 to formpartial pattern areas 18A to 18D. The partial pattern areas 18A to 18Deach has the same circuit pattern formed therein. Further, the partialpattern areas 18A to 18D are each provided with the same alignment mark19X for the X2-axis and the same alignment mark 19Y for the Y2-axis. TheX2 coordinate of the wafer stage 5B is measured by a combination of amoving mirror 6B and a laser interferometer 7B. The Y2 coordinate of thewafer stage 5B is measured by a combination of a moving mirror 8B and alaser interferometer 9B. The measured coordinates are supplied to acontroller 10B. The controller 10B controls the stepping drive of thewafer stage 5B.

[0146] The stepping drive of the wafer stage 5B is effected according toan array of shot areas (i.e. areas to each of which a pattern image ofthe pattern area 2B is to be projected by exposure) set on the exposuresurface of the wafer 20, that is, a shot map for a middle layer. Theshot map is generated by a map generating unit which comprises acomputer in the controller 10B. In this case, the map generating unit inthe controller 10A and the map generating unit in the controller 10Bhave the function of supplying shot map information prepared thereby toeach other. When exposure for a middle layer is to be carried out over acritical layer, for example, shot map information for the critical layerprepared by the map generating unit in the controller 10A of theexposure apparatus 1A is transmitted from a communication unit in thecontroller 10A to a communication unit in the controller 10B. The mapgenerating unit in the controller 10B generates a shot map for themiddle layer on the basis of the supplied shot map information.Conversely, when exposure for a critical layer is to be carried out overa middle layer, shot map information for the middle layer prepared bythe map generating unit in the controller 10B is supplied to the mapgenerating unit in the controller 10A.

[0147] In the exposure apparatus 1B also, an alignment system 11B forthe X2-axis is a TTL and LSA type alignment system. A laser beam fromthe alignment system 11B enters the projection optical system 3B via amirror 12B. The laser beam is converged through the projection opticalsystem 3B onto the wafer 20 in the form of a slit-shaped light spot 13Belongated in the direction Y2. A laser beam from an alignment system 14Bfor the Y2-axis enters the projection optical system 3B via a mirror15B, and the laser beam is converged through the projection opticalsystem 3B onto the wafer 20 in the form of a slit-shaped light spot 16Belongated in the direction X2. Diffracted light beams from theslit-shaped light spots 13B and 16B are received by the correspondingalignment systems 11B and 14B, thereby detecting the positions of thewafer marks for the Y2- and X2-axes on the wafer 20.

[0148] Next, an exposure method in this embodiment will be explainedwith reference to FIGS. 3 to 5. In this embodiment, the exposure processwill be explained by way of an example in which a pattern image of areticle for a middle layer is transferred by using the second exposureapparatus 1B over a critical layer transferred on the wafer 20 by usingthe first exposure apparatus 1A.

[0149]FIG. 3 shows a shot array of a critical layer on the wafer 20. InFIG. 3, the surface of the wafer 20 is divided into square shot areasSA₁₁, SA₁₂, . . . , SA₉₄ at a predetermined pitch in each of first andsecond directions. Each side of each square shot area SA has a length L.The shot areas SA₁₁ to SA₉₄ have approximately the same size as that ofthe exposure field 4A of the exposure apparatus 1A, shown in FIG. 1. Animage of the circuit pattern in the pattern area 2A of the reticle RA isprojected onto each of the shot areas SA₁₁ to SA₉₄ by using the exposureapparatus 1A, shown in FIG. 1. By development and other processescarried out thereafter, the circuit pattern images are made to appear asreal circuit patterns. Further, each of the shot areas SA_(ij) (i=1 to9; j=1 to 4) is provided with images of the alignment marks 17X and 17Yformed on the reticle RA, shown in FIG. 1, as a wafer mark MX_(ij) forthe X-axis and a wafer mark MY_(ij) for the Y-axis.

[0150] Next, a photoresist is coated over the wafer 20. The wafer 20coated with the photoresist is loaded onto the wafer stage 5B in theexposure apparatus 1B, shown in FIG. 1, and a circuit pattern image ofthe reticle RB is projected onto each of shot areas of a middle layerover the critical layer on the wafer 20. In this case, each group offour critical layer shot areas arrayed in two rows and two columns asshown in FIG. 3 corresponds to one middle layer shot area. For example,a group of four shot areas SA₁₁ to SA₁₄ in the top left cornercorresponds to one middle layer shot area SB₁. To generate such a shotmap for the middle layer, the exposure apparatus 1B first effects EGAalignment. The EGA alignment method is disclosed, for example, in JP(A)No. 4-277612 in addition to JP(A) No. 61-44429.

[0151] Here, the X2- and Y2-axes of the coordinate system that definethe travel position of the wafer stage 5B of the second exposureapparatus 1B, shown in FIG. 1, are taken as X- and Y-axes, respectively,in FIG. 3, and a coordinate system that is defined by the X- and Y-axesis referred to as “stage coordinate system (X,Y)”. From among thecritical layer shot areas SA₁₁ to SA₉₄ on the wafer 20 shown in FIG. 3,a predetermined number N (N is an integer of 3 or more) shot areas (i.e.shaded shot areas in the figure) are selected as sample shots S₁ to S₉(in this case, N=9), and coordinate values in the stage coordinatesystem (X,Y) of the wafer marks attached to the sample shots S₁ to S₉are measured by using the alignment systems 11B and 14B, shown inFIG. 1. For the sake of simplicity, it is assumed in the followingdescription that the X coordinate of the X-axis wafer mark MX_(ij)attached to a shot area SA_(ij) and the Y coordinate of the Y-axis wafermark MY_(ij) attached to the shot area SA_(ij) represent the X and Ycoordinates of the center of the shot area SA_(ij).

[0152] Further, coordinate axes which constitute the coordinate systemon the wafer 20 (i.e. sample coordinate system) are assumed to be anx-axis and a y-axis, respectively. It is further assumed that designcoordinate values of the centers of the shot areas SA₁₁ to SA₉₄ on thecritical layer in the sample coordinate system (x,y) have already beensupplied to the controller 10B of the second exposure apparatus 1B as apart of shot map data for the critical layer. Under these circumstances,the transformation of array coordinates of an arbitrary point on thewafer 20 in the sample coordinate system (x,y) into array coordinates inthe stage coordinate system (X,Y) is approximately expressed by thefollowing equation (3): $\begin{matrix}{\begin{bmatrix}X \\Y\end{bmatrix} = {{\begin{bmatrix}{Rx} & {- {{Rx}\left( {W + \Theta} \right)}} \\{{Ry} \cdot \Theta} & {Ry}\end{bmatrix}\begin{bmatrix}X \\{Y\quad}\end{bmatrix}} + \begin{bmatrix}{Ox} \\{Oy}\end{bmatrix}}} & (3)\end{matrix}$

[0153] The transformation matrix in Eq. (3) has as elements sixcoordinate transformation parameters, including scaling parameters Rxand Ry of the wafer, a rotation Θ [rad] of the shot array, aperpendicularity error W [rad] of the shot array, and offsets Ox and Oy.The scaling parameters Rx and Ry are linear expansion and contractionquantities of the wafer in the directions X and Y, respectively. Therotation Θ is an angle of rotation of the x-axis of the samplecoordinate system relative to the X-axis. The perpendicularity error Wis an error of the intersection angle between the x- and y-axes of thesample coordinate system from 90°. The offsets Ox and Oy are shiftquantities in the directions X and Y, respectively.

[0154] Eq. (3) is usable in the present invention; in this embodiment,however, Eq. (3) is approximated with the following equation (4) using1+Γx and 1+Γy for the scaling parameters Rx and Ry and regarding thevalues of the new parameters Γx and Γy as small in order to facilitatethe calculation: $\begin{matrix}{\begin{bmatrix}X \\Y\end{bmatrix} = {{\begin{bmatrix}{1 + {\Gamma \quad x}} & {- \left( {W + \Theta} \right)} \\\Theta & {1 + {\Gamma \quad y}}\end{bmatrix}\begin{bmatrix}x \\{y\quad}\end{bmatrix}} + \begin{bmatrix}{Ox} \\{Oy}\end{bmatrix}}} & (4)\end{matrix}$

[0155] To determine values of the six transformation parameters (Γx, Γy,Θ, W, Ox and Oy) in Eq. (4), the controller 108 defines the arraycoordinate values of the centers (wafer marks) of sample shots S_(i)measured by the i-th (i=1 to N) measuring operations as (XM_(i),YM_(i)).Next, the design array coordinates (x_(i),y_(i)) of the centers of thesample shots S_(i) are substituted for the coordinates (x,y) on theright-hand side of Eq. (4) to obtain computational array coordinatevalues (X_(i),Y_(i)). The sum of the squares of deviations of themeasured values (XM_(i),YM₁) from the array coordinate values(X_(i),Y_(i)) is determined to be a residual error component asexpressed by the following equation (5): $\begin{matrix}{{{Residual}\quad {error}\quad {component}} = {\sum\limits_{i = 1}^{N}\left\{ {\left( {X_{i} - {XM}_{i}} \right)^{2} + \left( {Y_{i} - {YM}_{i}} \right)^{2}} \right\}}} & (5)\end{matrix}$

[0156] Then, the controller 10B determines values of the sixtransformation parameters so that the residual error component isminimized. For example, values of the six parameters are obtained bysolving simultaneous equations established by setting the result ofpartial differentiation of the right-hand side of Eq. 5 with respect toeach of the six parameters equal to zero.

[0157] In this embodiment, it is assumed that among the transformationparameters obtained as described above, the shot array rotation Θ [rad]is regarded as zero, and the shot array perpendicularity error W [rad]assumes a predetermined finite value. This means that a perpendicularityerror W exits in the critical layer shot array. The other parameters,that is, scaling parameters Γx and Γy and offsets Ox and Oy, may assumeany values, respectively. In this case, the array of the critical layershot areas SA₁₁ to SA₉₄ is as follows: As shown in FIG. 3, for example,an imaginary straight line 23 connecting the centers of shot areas whichare adjacent to each other in the direction X is parallel to the X-axis.An imaginary straight line 24 which passes through the center C₁₁ of thefirst shot area SA₁₁ and which connects the centers of shot areas whichare successively adjacent to the shot area SA₁₁ in the direction Y hasbeen rotated clockwise relative to the Y-axis by the perpendicularityerror W.

[0158] Next, in this embodiment, the reticle RB in the exposureapparatus 1B, shown in FIG. 1, is rotated through a predetermined angleδ [rad] to thereby rotate each shot area of the middle layer by an angleδ in order to reduce an overlay error between the critical and middlelayers due to the perpendicularity error W.

[0159]FIG. 4 shows the positional relationship between four shot areasSA₁₁ to SA₁₄ of the critical layer and one middle layer shot area SB₁over the four critical layer shot areas. In FIG. 4, the shot area SB₁has its adjacent sides rotated through an angle δ clockwise from therespective positions which are parallel to the X- and Y-axes. Further,the controller 10B successively substitutes the design array coordinatesof the four shot areas SA₁₁ to SA₁₄ and the above-determinedtransformation parameters into the right-hand side of Eq. (4), therebyobtaining center coordinates of the four shot areas SA₁₁ to SA₁₄ in thestage coordinate system (X,Y), and further obtaining coordinates of thecenter 25 of the four sets of center coordinates. Then, a circuitpattern image of the reticle RB is projected onto the shot area SB₁ withthe center 25 made coincident with the center of the exposure field 4B.As a result, exposure is carried out in a state where the center 25 ofthe array of the four shot areas SA₁₁ to SA₁₄ is coincident with thecenter of the shot area SB₁, and the shot area SB₁ has been rotatedthrough the angle δ.

[0160] Similarly, as shown in FIG. 5, a circuit pattern image of thereticle RB is sequentially projected onto middle layer shot areas SB₂,SB₃, . . . , SB₉ deployed over the critical layer on the wafer 20.

[0161] Let us conduct evaluation of the overlay error in this embodimentwith reference to FIG. 4. In this embodiment, in an array of four squareshot areas SA₁₁ to SA₁₄, each side of which has a length L, shot areaswhich are adjacent to each other in the direction X lie such that animaginary straight line 23A connecting the centers of these shot areasis parallel to the X-axis, and shot areas which are adjacent to eachother in the direction Y lie such that an imaginary straight line 24connecting the centers of these shot areas intersects the Y-axis at anangle (perpendicularity error) W. Accordingly, assuming that an overlayerror between the array of critical layer four shot areas SA₁₁ to SA₁₄and the middle layer shot area SB₁ is Δ, and that the perpendicularityerror W and the rotation angle δ are small, the ranges of X and Ycomponents Δ_(x) and Δ_(y) of the overlay error Δ concerning the shotarea SA₁₁ are approximately given by the following equation (6):

(½)L·W−L·δ≦Δ _(X)≦(½)L·W 0≦Δ_(Y) ≦L·δ  (6)

[0162] In this case, if the rotation angle δ is assumed to be zero as inthe related art shown in FIG. 35, the X component Δ_(x) of the overlayerror Δ is uniformly (½)L·W, and the Y component Δ_(y) is uniformlyzero. Therefore, in order to reduce the overlay error to a lower levelthan in the related art as a whole, the rotation angle δ should be setwithin the range given by the following equation (7):

0<δ<(½)W  (7)

[0163] The value of the rotation angle δ at which the overlay errorreaches a minimum as a whole within the above range is (¼)W. That is, inthis case, the ranges of the X and Y components are obtained from Eq.(6) as follows:

(¼)L·W≦Δ _(X)≦(½)L·W

0≦Δ_(Y)≦(¼)L·W

[0164] Thus, it becomes possible to regard the overlay error as minimumas a whole.

[0165] Although in the above-described embodiment the reticle-side partin the exposure apparatus 1B is rotated through the rotation angle δ,the wafer-side part may be rotated through −δ instead of rotating thereticle-side part. However, if the wafer-side part is rotated, the shotarea array on the critical layer also changes, and it is thereforenecessary to correct the critical layer shot array. That is, rotatingthe reticle-side part is advantageous because it is possible to omit acorrective calculation which would otherwise be required.

[0166] Although in this embodiment the perpendicularity error W of theshot array is assumed to be not zero, it should be noted that analignment method similar to that in the above-described embodiment isalso applicable in a case where the perpendicularity error W is zero asin the related art shown in FIG. 36 and the shot rotation θ of each ofthe critical layer shot areas SA₁₁ to SA₉₄, shown in FIG. 3, is notzero. That is, the overlay error can be reduced as a whole by rotatingthe reticle-side part of the second exposure apparatus 1B, for example,such that each middle layer shot area is rotated through the angle(θ+δ′) relative to the corresponding array of four critical layer shotareas, using the angle δ′ in the range of 0<δ′<(½)θ. In particular, ifthe angle δ′ is set at (¼)θ, the overlay error is minimized as a whole.The method of detecting the shot rotation θ will be explained in asecond example of the first embodiment.

[0167] Next, the second example of the first embodiment of the presentinvention will be described with reference to FIGS. 6(a) to 8(c). In thesecond example, the stepper type exposure apparatus 1A, shown in FIG. 1,is used as a first exposure apparatus, and a step-and-scan type scanningexposure apparatus with a demagnification ratio of 4:1 is used as asecond exposure apparatus. In the first exposure apparatus, an image oftwo identical circuit patterns (chip patterns) is transferred per shotarea; in the second exposure apparatus, an image of three identicalcircuit patterns is transferred per shot area.

[0168]FIG. 6(a) shows a part of a shot array on a wafer loaded on awafer stage (not shown) of the scanning type second exposure apparatusin this example. In FIG. 6(a), square shot areas SA₁, SA₂ and SA₃, eachside of which has a length L, are sequentially arrayed, lying adjacentto each other in the direction Y. The shot areas SA₁, SA₂, and SA₃ eachhas two identical circuit patterns 26A and 26B formed side-by-side inthe direction Y by the first exposure apparatus, a developer, etc. InFIG. 6(a), X- and Y-axes represent a stage coordinate system of thesecond exposure apparatus. An imaginary straight line 27 passing throughthe centers of the shot areas SA₁, SA₂ and SA₃ on the first layer istilted by an angle W clockwise relative to the Y-axis. The angle W is aperpendicularity error of the shot array.

[0169]FIG. 6(b) shows a shot area SC which has a width L in thedirection X and a width (3/2)L in the direction Y on a wafer which is tobe exposed by the second exposure apparatus in this example. In FIG.6(b), directions +Y and −Y are scanning directions. That is, the shotarea SC is scanned in the direction +Y, for example, relative to aslit-shaped illumination field 28, and a reticle placed through aprojection optical system is scanned in the direction −Y in synchronismwith the scanning of the shot area SC. As a result, three identicalcircuit pattern images 29A to 29C are formed on the shot area SC, lyingside-by-side in the direction Y. In this example, it is assumed thatcircuit pattern images for two shot areas SC₁ and SC₂ each having thesame size as that of the shot area SC shown in FIG. 6(b) are overlaid onthe shot areas SA₁, SA₂ and SA₃ shown in FIG. 6(a) by using the secondexposure apparatus.

[0170] In this case, it is conceivable to effect alignment such that, asshown by the chain double-dashed lines in FIG. 6(a), reference points 27a and 27 b, which are at the centers of two arrays of three circuitpatterns arranged in the direction Y, coincide with the centers ofsecond-layer shot areas SC₁ and SC₂, respectively, on an imaginarystraight line 27 passing through the centers of the first-layer shotareas SA₁, SA₂ and SA₃. However, this alignment method causes overlayerrors a and b in the direction X between the second-layer shot area SC₁and the first-layer shot areas SA₁ and SA₂. Similarly, overlay errors band a in the direction X arise between the second-layer shot area SC₂and the first-layer shot areas SA₂ and SA₃. The overlay errors a and bare expressed by the following equation (8):

a=(¼)L·W b=(¾)L·W (8)

[0171] Accordingly, it will be understood that the alignment methodshown in FIG. 6(a) causes a large overlay error b to arise particularlyin the second shot area SA₂. In order to reduce the overlay error, inthis example, the center positions of the second-layer shot areas SC₁and SC₂ are shifted by a predetermined distance in the direction X fromthe reference points 27 a and 27 b, respectively.

[0172]FIG. 7 shows the alignment method according to this example. InFIG. 7, the center positions of the second-layer shot areas SC₁ and SC₂are shifted by a distance d in the respective directions −X and +Xrelative to the reference points 27 a and 27 b on the imaginary straightline 27 passing through the centers of the first-layer shot areas SA₁ toSA₃. As a result, there is a uniform overlay error c in the direction Xbetween the first-layer shot areas SA₁ to SA₃ and the second-layer shotareas SC₁ and SC₂. The distance d and the overlay error c are given bythe following equation (9):

d=(¼)L·W c=(½)L·W  (9)

[0173] As a result, the overlay error c given by Eq. (9) is (½)L·W incontrast to the overlay error b given by Eq. (8). Accordingly, it willbe understood that the alignment method according to this exampleenables the maximum value of the overlay error to reduce to (½)L·W, andthus the overlay error reduces as a whole. It should be noted thatduring the alignment shown in FIG. 7, the second-layer shot areas SC₁and SC₂ may be rotated through a predetermined angle relative to thefirst-layer shot areas SA₁ to SA₃ by additionally applying the methodaccording to the first example. By doing so, the overlay error may befurther reduced as a whole.

[0174] In the second example of the first embodiment, exposure may becarried out by using the first exposure apparatus over shot areasexposed by using the scanning type second exposure apparatus in reverserelation to the above-described exposure operation. One example of suchan exposure operation will be explained below with reference to FIGS.8(a), 8(b) and 8(c).

[0175]FIG. 8(a) shows the first-layer shot areas SC₁ and SC₂ on thewafer which have been formed with circuit patterns by using the scanningsecond exposure apparatus. In FIG. 8(a), the shot areas SC₁ and SC₂ eachhas three identical circuit patterns arranged in the direction Y, andthere is a predetermined perpendicularity error in the shot array. Then,a reticle pattern image including two identical circuit pattern imagesarranged in the direction Y is formed over the shot areas SC₁ and SC₂for each of the shot areas SA₁, SA₂ and SA₃ by using the first exposureapparatus. In this case, for the first and third shot areas SA₁ and SA₃,it is only necessary to align them with the first-layer shot areas SC₁and SC₂, respectively, in the direction X. For the second shot area SA₂,it is only necessary to align its position in the direction X with anintermediate position between the first-layer shot areas SC₁ and SC₂.Consequently, the overlay error between the first and second layers is bonly at the shot area SA₂.

[0176] However, in a case where a reticle pattern image is transferredby using the first exposure apparatus, if a part of the pattern image tobe transferred can be selectively masked by using a reticle blind(variable field stop), for example, the overlay error can be reduced toapproximately zero. In such a case, as shown in FIG. 8(b), when thesecond shot area SA₂ of the second layer is to be exposed, first, theposition in the direction X of the shot area SA₂ is aligned with thefirst-layer shot area SC₁. Thereafter, exposure is carried out with thelower half of the shot area SA₂ masked by controlling the reticle blind.Consequently, exposure is effected only for the upper half of the shotarea SA₂, which corresponds to the circuit pattern 26A.

[0177] Next, the position in the direction X of the shot area SA₂ isaligned with the first-layer shot area SC₂, and thereafter, exposure iscarried out with the upper half of the shot area SA₂ masked bycontrolling the reticle blind. Consequently, exposure is effected onlyfor the lower half of the shot area SA₂, which corresponds to thecircuit pattern 26B. For the other shot areas SA₁ and SA₃, exposuresimilar to that in the case of FIG. 8(a) is carried out. As a result,the overlay error becomes zero at all the shot areas.

[0178] Next, a third example of the first embodiment of the presentinvention will be described with reference to FIGS. 9(a) to 10(c). Inthis example, the stepper type exposure apparatus 1A, shown in FIG. 1,is used as a first exposure apparatus, and a step-and-scan type scanningexposure apparatus with a demagnification ratio of 4:1 is used as asecond exposure apparatus. In this case, shot areas as exposure unitswhich are to be exposed by the first exposure apparatus each containstwo identical square circuit patterns, each side of which has a lengthL. Shot areas as exposure units which are to be exposed by the secondexposure apparatus each contains three identical rectangular circuitpatterns in which one side has a length L, and the other side has alength 3L/2.

[0179]FIG. 9(a) shows a shot array on a wafer 20 loaded on a wafer stage(not shown) of the scanning type second exposure apparatus. In FIG.9(a), square shot areas SA₁, SA₂, SA₃, . . . , SA₆₆, each side of whichhas a length L, are arranged at a predetermined pitch in each of thedirections X and Y. The shot areas SA₁, SA₂, . . . each has twoidentical circuit patterns 26A and 26B formed to lie side-by-side in adirection substantially parallel to the direction Y by the firstexposure apparatus, a developer, etc. In FIG. 9(a), X- and Y-axesrepresent a stage coordinate system of the second exposure apparatus.Further, the shot areas SA₁, SA₂, . . . have been each formed with twowafer marks MYA_(i) and MYB_(i) for the Y-axis and two wafer marksMXA_(i) and MXB_(i) for the X-axis, which are detectable by an LSA typedetection method. In FIG. 9(a), only the four wafer marks MYA₁, MYB₁,MXA₁ and MXB₁ in the shot area SA₁ are shown for the sake of simplicity.

[0180] In this example also, alignment is effected by the EGA method inthe same way as in the first example. In this example, however, eachshot area contains four one-dimensional wafer marks, and therefore, twoin-shot transformation parameters can be obtained in addition to theabove-described six transformation parameters (i.e. scaling parametersRx and Ry, shot array rotation Θ, shot array perpendicularity error W,and offsets Ox and Oy).

[0181] Accordingly, in this example, a shot rotation (chip rotation) θ[rad] and a shot perpendicularity error w [rad] are obtained as in-shottransformation parameters. It should be noted that shot magnificationsrx and ry can also be obtained by disposing two other one-dimensionalwafer marks in each shot area. However, this example is not particularlyrelated to the determination of shot magnifications rx and ry;therefore, wafer marks for them are not provided in this example. Amethod in which EGA alignment is effected by using three or moreone-dimensional wafer marks or two or more two-dimensional wafer marks,which are disposed in each shot area, as described above, is also knownas “in-shot multipoint EGA alignment method”.

[0182] More specifically, in this example, a predetermined number N (Nis an integer of 3 or more) of shot areas are selected from among shotareas SA₁ to SA₆₆ on a wafer 20 as sample shots S₁ to S₉ (in this case,N=9), and coordinate values in the stage coordinate system (X,Y) of twopairs of wafer marks attached to each of the sample shots S₁ to S₉ aremeasured by using an LSA type alignment system. For example, a meanvalue of the X coordinates of the two X-axis wafer marks of each sampleshot and a mean value of the Y coordinates of the two Y-axis wafer marksof the sample shot are regarded as array coordinates of the center ofthe sample shot, thereby obtaining parameters (i.e. scaling parametersRx and Ry, rotation Θ, perpendicularity error W, and offsets Ox and Oy)for transformation from the sample coordinate system (x,y) into thestage coordinate system (X,Y) in the same way as in the first example.

[0183] Further, in this example, a shot rotation θ, which is a rotationangle δf the x-axis in a shot, is calculated on the basis of a meanvalue of Y-coordinate differences between the pairs of Y-axis wafermarks of the sample shots, for example, and a rotation angle θ_(y) ofthe y-axis in a shot is calculated on the basis of a mean value ofX-coordinate differences between the pairs of X-axis wafer marks. Theshot rotation θ is subtracted from the rotation angle θ_(y) of they-axis to obtain an angle w, which is determined to be a shotperpendicularity error.

[0184] It is assumed in this example that, as a result of the alignment,as shown in FIG. 9(a), the shot array rotation Θ, the shot arrayperpendicularity error W and the shot perpendicularity error w havebecome capable of being regarded as zero, but the shot rotation θ hasbecome a predetermined value other than zero. On such a shot array,overlay exposure is effected by the scanning second exposure apparatushaving a shot area SC₁ as shown in FIG. 9(b), which has a size (3L/2)that is sufficiently large to contain three circuit patterns in thedirection Y as a scanning direction, and which has a width L in thedirection X. The method of alignment effected to carry out the overlayexposure will be explained below.

[0185]FIG. 10(a) shows a part of the shot array on the wafer 20 shown inFIG. 9(a). In FIG. 10(a), square shot areas SA₁, SA₂ and SA₃, each sideof which has a length L, have been rotated counterclockwise through anangle corresponding to the shot rotation θ. It is assumed that circuitpattern images for two shot areas SC₁ and SC₂, each having the same sizeas the shot area SC shown in FIG. 9(b), are overlaid on the shot areasSA₁ to SA₃ by using the second exposure apparatus.

[0186] In this case, it is conceivable to effect alignment such that, asshown by the chain double-dashed lines in FIG. 10(a), reference points31 a and 31 b, which are at the centers of two arrays of three circuitpatterns, coincide with the centers of the second-layer shot areas SC₁and SC₂, respectively, on an imaginary straight line 31 passing throughthe centers of the first-layer shot areas SA₁, SA₂ and SA₃ in parallelto the Y-axis. In this case, if the scanning direction in the secondexposure apparatus is restricted to the directions +Y and −Y, it isnecessary to rotate the wafer or the reticle clockwise through the angleθ, for example. Further, if the wafer is rotated, it is necessary tocorrect the array coordinates of each of the first-layer shot areas. Inthe following description, the scanning direction is assumed to be adirection intersecting the Y-axis at the angle θ.

[0187] With this alignment method, however, X-direction overlay errors aand b, which are given by the following equation (10), arise between thesecond-layer shot area SC₁ and the first-layer shot areas SA₁ and SA₂,respectively, in the same way as in a case where there is a shot arrayperpendicularity error as shown in FIG. 6(a):

a=(¼)L·θ b=(¾)L·θ  (10)

[0188] Accordingly, it will be understood that the alignment method asshown in FIG. 10(a) causes a large overlay error b to arise particularlyat the second shot area SA₂. In order to reduce the overlay error, inthis example, the center positions of the second-layer shot areas SC₁and SC₂ are shifted by a predetermined distance from the referencepoints 31 a and 31 b in a direction perpendicular to the scanningdirection.

[0189]FIG. 10(b) shows an alignment method carried out in this example.In FIG. 10(b), the center positions of the second-layer shot areas SC₁and SC₂ have been shifted by −d and +d relative to the reference points31 a and 31 b in a direction intersecting the X-axis at the angle θ inthe clockwise direction. As a result, there is a uniform overlay error cin the direction X between the first-layer shot areas SA₁ to SA₃ and thesecond-layer shot areas SC₁ and SC₂. The distance d and the overlayerror c are given by the following equation (11):

d=(¼)·θ c=(½)L·θ  (11)

[0190] As a result, the overlay error c given by Eq. (11) is (½)L·θ incontrast to the overlay error b given by Eq. (10). Accordingly, it willbe understood that the alignment method according to this exampleenables the maximum value of the overlay error to reduce to (½)L·θ, andthus the overlay error reduces as a whole.

[0191] In the third example of the first embodiment, exposure may becarried out by using the first exposure apparatus over shot areasexposed by using the scanning type second exposure apparatus in reverserelation to the above-described exposure operation. One example of suchan exposure operation will be explained below with reference to FIG.10(c).

[0192]FIG. 10(c) shows the first-layer shot areas SC₁ and SC₂ on thewafer which have been formed with circuit patterns by using the scanningsecond exposure apparatus. In a case where, in FIG. 10(c), a reticlepattern image is to be formed for each of the shot areas SA₁, SA₂ andSA₃ over the shot areas SC₁ and SC₂ by using the first exposureapparatus, for the first and third shot areas SA₁ and SA₃, it is onlynecessary to align them with the first-layer shot areas SC₁ and SC₂,respectively, in the direction X. For the second shot area SA₂, it isonly necessary to align its position in a direction perpendicular to thescanning direction with an intermediate position between the first-layershot areas SC₁ and SC₂. Consequently, the overlay error between thefirst and second layers is b only at the shot area SA₂.

[0193] Further, for the second shot area SA₂, exposure may be effectedfor the upper and lower halves separately in the same way as in themethod described with reference to FIGS. 8(b) and 8(c). By doing so, theoverlay error can be reduced to zero.

[0194] Although in the third example, only the shot rotation θ iscorrected, it should be noted that alignment may be effected as follows:A mean value of the shot rotation θ obtained by the in-shot multipointEGA method and the shot perpendicularity error w, i.e. (θ+w)/2, isregarded as pseudo shot rotation, and alignment is effected on the basisof the pseudo shot rotation.

[0195] Further, although in the third example the shot rotation θ isobtained by measuring the positions of three or more wafer marks in eachsample shot, the shot rotation θ may be obtained by using numericalvalues previously obtained by test printing using the first exposureapparatus. In this case, in each sample shot the ordinary EGA typealignment is effected by measuring the positions of a pair of wafermarks as in the conventional practice, and the shot rotation θ alone isobtained by using the input numerical values.

[0196] Although in the above-described embodiment two steppers or acombination of a stepper and a step-and-scan type projection exposureapparatus is used, it should be noted that, for example, twostep-and-scan type projection exposure apparatuses may be used as twoexposure apparatuses having respective exposure fields of differentsizes.

[0197] The exposure method according to the first embodiment of thepresent invention provides the following advantages. In a case whereexposure is carried out by the mix-and-match method using two exposureapparatuses having respective exposure fields of different sizes, aperpendicularity error in the shot area array on the preceding layer ora mean value of rotation angles of the shot areas is detected, and theshot areas of the subsequent layer are rotated according to the resultof the detection. Accordingly, the overlay error between the two layerscan be favorably reduced.

[0198] Further, in the exposure method according to the first embodimentof the present invention, when exposure is to be carried out by themix-and-match method using two exposure apparatuses having respectiveexposure fields of different sizes, a perpendicularity error in the shotarea array on the preceding layer or a mean value of rotation angles ofthe shot areas is detected, and exposure is carried out with the shotareas of the subsequent layer shifted in a direction perpendicular tothe scanning direction of the second exposure apparatus according to theresult of the detection. Accordingly, the overlay error between the twolayers can be favorably reduced.

[0199] In this case, if the shot areas of the subsequent layer arerotated in addition to the shifting of the shot areas, the overlay errormay be further reduced.

[0200] Next, a first example of a second embodiment of the exposuremethod according to the present invention will be described withreference to FIGS. 11 to 14(c). Two exposure apparatuses used in thisexample are a one-shot exposure type projection exposure apparatus(stepper) with a demagnification ratio of 5:1 and a step-and-scan typeprojection exposure apparatus with a demagnification ratio of 4:1. Inthis example, two chip patterns are formed in each shot area exposed bythe former projection exposure apparatus (i.e. a two-chip reticle isused), and three chip patterns are formed in each shot area scan-exposedby the latter projection exposure apparatus (i.e. a three-chip reticleis used). It should be noted that constituent elements in the secondembodiment which are similar to those in the first embodiment aredenoted by the same reference characters.

[0201]FIG. 11 shows an exposure system used in this example. In FIG. 11,a stepper 1A, which is a one-shot exposure type projection exposureapparatus, and a step-and-scan type projection exposure apparatus(hereinafter referred to as “scanning exposure apparatus”) 1B areinstalled. In this example, the stepper 1A is a high-resolution exposureapparatus, while the scanning exposure apparatus 1B is a low-resolutionexposure apparatus. The stepper 1A is used to carry out exposure for acritical layer, which requires high resolution, on a wafer, and thescanning exposure apparatus 1B is used to carry out exposure for amiddle layer, which does not require high resolution, on the wafer.However, the stepper 1A may be a low-resolution exposure apparatus orthe scanning exposure apparatus 1B may be a high-resolution exposureapparatus according to the kind of semiconductor device to be produced.

[0202] First, in the stepper 1A, a pattern area 42A on a reticle RA isilluminated by exposure light from an illumination optical system (notshown), and an image of a pattern formed in the pattern area 42A isformed on a rectangular exposure field 44A on a wafer 20 as a projectedimage reduced to 1/5 by a projection optical system 3A. A Z1-axis istaken in a direction parallel to an optical axis of the projectionoptical system 3A, and two axes of an orthogonal coordinate system setin a plane perpendicular to the Z1-axis are defined as an X1-axis and aY1-axis, respectively. The pattern area 42A on the reticle RA is dividedinto partial pattern areas 112A and 112B of the same size in apredetermined direction (in FIG. 11, in the direction Y1). The partialpattern areas 112A and 112B each has original drawing patterns ofcircuit patterns and alignment marks arranged according to the samelayout.

[0203] The wafer 20 is held on a wafer stage 5A. The wafer stage 5Acomprises a Z-stage for moving the wafer 20 in the direction Z1 to setan exposure surface of the wafer 20, which is to be exposed, at the bestfocus position, and an XY-stage for positioning the wafer 20 in both thedirections of the X1- and Y1-axes. A pair of moving mirrors 6A and 8Awhich are perpendicular to each other are fixed on the wafer stage 5A.The coordinate in the direction X1 of the wafer stage 5A is measured bya combination of the moving mirror 6A and a laser interferometer 7Awhich is installed outside the wafer stage 5A. The coordinate in thedirection Y1 of the wafer stage 5A is measured by a combination of themoving mirror 8A and a laser interferometer 9A which is installedoutside the wafer stage 5A. The coordinates measured by the laserinterferometers 7A and 9A are supplied to a controller 10A whichcontrols operations of the whole apparatus. The controller 10A drivesthe wafer stage 5A to step in both the directions X1 and Y1 throughdrive units (not shown), thereby positioning the wafer 20. In this case,the stepping drive of the wafer 20 is effected according to an array ofshot areas (i.e. unit areas to each of which a pattern image of thepattern area 42A is to be projected by exposure) set on the exposuresurface of the wafer 20, that is, a shot map for a critical layer. Theshot map is generated by a map generating unit which comprises acomputer in the controller 10A. It is assumed that a predeterminedperpendicularity error W remains in a coordinate system (i.e. stagecoordinate system) (X1,Y1) which defines the travel position of thewafer stage 5A of the stepper 1A in this example.

[0204] Further, the stepper 1A in this example is provided with anoff-axis imaging type (FIA type) alignment system 11A. The alignmentsystem 11A images an alignment mark (wafer mark) on the wafer 20 andprocesses an imaging signal thus obtained to detect X1 and Y1coordinates of the wafer mark. The detected coordinates are supplied tothe controller 10A.

[0205] It should be noted that, as the alignment system 11A, it is alsopossible to use a TTR (Through-The-Reticle) type alignment system or aTTL (Through-The-Lens) type alignment system in which the position of amark is detected through the projection optical system 3A. As a markdetecting method, it is also possible to use a laser step alignment(LSA) method in which a slit-shaped laser beam and a mark are scannedrelative to each other, or a so-called two-beam interference method (LIAmethod) in which two light beams are applied to a diffractiongrating-shaped mark, and the position of the mark is detected from asignal obtained from interference between a pair of diffracted lightbeams generated in parallel from the illuminated mark.

[0206] Next, in the scanning exposure apparatus 1B in this example, apart of a pattern area 42B on a reticle RB is illuminated by exposurelight from an illumination optical system (not shown), and an image of apart of the reticle pattern is formed in a slit-shaped exposure area 144on a wafer 20, which is held on a wafer stage 5B, as a projected imagereduced to ¼ by a projection optical system 3B. Here, a Z2-axis is takenin a direction parallel to an optical axis of the projection opticalsystem 3B, and two axes of an orthogonal coordinate system set in aplane perpendicular to the Z2-axis are defined as an X2-axis and aY2-axis, respectively. Under these circumstances, the reticle RB isscanned in the direction −Y2 (or +Y2), and the wafer 20 is scanned inthe direction +Y2 (or −Y2) in synchronism with the scanning of thereticle RB, thereby sequentially projecting an image of the patternformed in the pattern area 42B of the reticle RB onto the exposure field44B on the wafer 20.

[0207] The pattern area 42B of the reticle RB is divided into threepartial pattern areas 13A to 13C of the same size in the direction Y2,which is the scanning direction. The size of the exposure field 44B issuch that its dimension in the scanning direction is 3/2 times as largeas the dimension of the exposure field 44A of the stepper 1A, and theexposure field 44B is equal in size (1:1) to the exposure field 44A inthe non-scanning direction. That is, the exposure field 44B is longerthan the exposure field 44A in the direction Y2.

[0208] The position of a reticle stage (not shown) for scanning thereticle RB of the scanning exposure apparatus 1B is measured by a laserinterferometer (not shown). The X2 coordinate of the wafer stage 5B ismeasured by a combination of a moving mirror 6B and a laserinterferometer 7B, and the Y2 coordinate of the wafer stage 5B ismeasured by a combination of a moving mirror 8B and a laserinterferometer 9B. The measured coordinates of the wafer stage 5B aresupplied to a controller 10B. In this example, the X2- and Y2-axes areassumed to be perpendicular to each other. The controller 10B controlssynchronous drive of the reticle stage (not shown) and the wafer stage5B. Scanning exposure of the wafer stage 5B is effected according to ashot map for a middle layer set on an exposure surface of the wafer 20,which is to be exposed. The shot map is generated by a map generatingunit which comprises a computer in the controller 10B.

[0209] In this case, the map generating unit in the controller 10A andthe map generating unit in the controller 10B have the function ofsupplying shot map information prepared thereby to each other. Whenexposure for a middle layer is to be carried out over a critical layer,for example; shot map information for the critical layer prepared by themap generating unit in the controller 10A of the stepper 1A istransmitted to the other controller 10B. The map generating unit in thecontroller 10B generates a shot map for the middle layer on the basis ofthe supplied shot map information. Conversely, when exposure for acritical layer is to be carried out over a middle layer, shot mapinformation for the middle layer prepared in the controller 10B issupplied to the controller 10A.

[0210] The scanning exposure apparatus 1B also has an off-axis imagingtype (FIA type) alignment system 11B provided at a side surface of theprojection optical system 3B. The alignment system 11B detects X2 and Y2coordinates of a wafer mark on the wafer 20.

[0211] Next, one example of an exposure operation which is performed inthis example when exposure for a first-layer pattern is first effectedby using the stepper 1A and then exposure for a second-layer pattern iseffected by using the scanning exposure apparatus 1B will be explainedfor each of the first and second processing steps.

[0212] First, the first step will be explained.

[0213] In the first step, as shown in FIG. 12(a), the reticle RA isfixed on the reticle stage (not shown) of the stepper 1A, shown in FIG.11, such that the reticle RA is rotated through 90° from its ordinaryposition. As a result, the two partial pattern areas 112A and 112B inthe pattern area 42A of the reticle RA lie side-by-side in the directionX1. Next, as shown in FIG. 12(b), the wafer 20 coated with a photoresistis fixed on the wafer stage 5A of the stepper 1A, shown in FIG. 11, suchthat the wafer 20 is rotated through 90° from its ordinary position. Asa result, the wafer 20 is placed such that the cut portion (orientationflat) of the outer periphery of the wafer 20 faces in the direction +X1.Although in FIG. 12(b) the mutual origin of the X1 and Y1-axes is set atthe center of the wafer 20, in FIG. 12(a) the origin of the two axes isset outside the reticle RA for the sake of explanation. Further, in FIG.12(a), the reticle RA is shown in the size of an image thereof asprojected on the wafer 20. In this example, as shown in FIG. 12(b), theY1-axis has rotated through an angle W clockwise relative to animaginary axis Y1* perpendicularly intersecting the X1-axis; the angle Wis a perpendicularity error.

[0214] Next, a pattern image of the reticle RA is sequentially projectedonto shot areas 121A, 121B, . . . , 121I, which are obtained by dividinga first-layer exposure area on the wafer 20 at a predetermined pitch ineach of the directions X1 and X1, by the step-and-repeat method usingthe stepper 1A. For example, the first-layer shot areas 121A to 121I arearranged in an array of 3 columns in the direction X1 and 3 rows in thedirection Y1. In this case, there is the perpendicularity error Wbetween the X1- and Y1-axes; therefore, the array of shot areas 121A to121I also has the perpendicularity error W.

[0215] However, in this example, exposure has been effected with thereticle RA and the wafer 20 each rotated through 90°. Therefore, in FIG.12(b), the first shot area 121A on the wafer 20 has two partial shotareas 122A and 122B divided in the direction X1. The-partial shot areas122A and 122B have been exposed to pattern images which are identicalwith each other. The same is the case with the other shot areas 121B to121I. When edges of the shot areas 121A to 121C in the first row whichare parallel to the array direction of the partial shot areas 122A and122B are connected together, a straight line 125, which has noirregularity, is obtained.

[0216] Thereafter, the wafer 20 is subjected to development, therebyallowing the circuit pattern images and alignment mark images in eachshot area to appear as circuit patterns and wafer marks, which compriserecess-and-projection patterns. In this example, the first partial shotarea 122A in the first shot area 121A is formed with an X-axis wafermark 123X and a Y-axis wafer mark 123Y, which comprise line-and-spacepatterns, respectively. The second partial shot area 122B is also formedwith an X-axis wafer mark 124X and a Y-axis wafer mark 124Y. The wafermarks 123X, 123Y, 124X and 124Y are marks which are detectable with animaging type alignment sensor. It should be noted that the arrangementof wafer marks is not necessarily limited to the example shown in FIG.12(b). For example, the arrangement of wafer marks may be such that apair of wafer marks are disposed in each of the shot areas 121A to 121I.It is also possible to dispose more than one pair of wafer marks in eachof the shot areas 121A to 121I. Further, two-dimensional marks may beused as wafer marks.

[0217] Next, the second step will be explained.

[0218] A photoresist is coated over the wafer 20 having the circuitpatterns and wafer marks formed thereon in the first step. Thephotoresist-coated wafer 20 is fixed at the ordinary rotation angle onthe wafer stage 5B of the scanning exposure apparatus 1B, shown in FIG.11. Thus, as shown in FIG. 13, the wafer 20 is disposed such that itscut portion faces in the direction −Y2. As shown in FIG. 11, the reticleRB for the second layer is also set at the ordinary rotation angle, thatis, at an angle at which the partial pattern areas 113A to 113C arearranged in the direction Y2.

[0219] At this time, the first-layer shot array data is supplied fromthe controller 10A of the stepper 1A to the controller 10B of thescanning exposure apparatus 1B. The controller 10B determines asecond-layer shot array on the basis of the supplied shot array data,together with alignment data (described later).

[0220] Thereafter, the wafer 20, which is an object to be exposed, issubjected to alignment by the EGA method in the scanning exposureapparatus 1B.

[0221]FIG. 13 shows the wafer 20 as an object to be exposed. In FIG. 13,the origin of the stage coordinate system (X2,Y2) of the scanningexposure apparatus 1B is set at the center of the wafer 20 for the sakeof convenience. The origin of the stage coordinate system (X1,Y1) of thestepper 1A used to expose the first layer is also shown to be coincidentwith the center of the wafer 20. In this case, because the wafer 20 isset at the ordinary rotation angle, the two partial shot areas 122A and122B in the shot area 121A, for example, are arranged in the directionY2, and in each of the other shot areas 121B to 121I, the two partialshot areas are also arranged in the direction Y2. To effect EGA typealignment, three or more shot areas are selected as sample shots fromamong the nine shot areas 121A to 121I on the wafer 20, and thecoordinates in the stage coordinate system (X2,Y2) of the wafer marks inthe sample shots are measured by using the alignment system 11B, shownin FIG. 11. When the shot area 121A, for example, is selected as asample shot, the coordinates of a pair of wafer marks 123X and 123Y inthe first partial shot area 122A of the shot area 121A are measured, andfor each of the other sample shots also, the coordinates of a pair ofwafer marks are measured.

[0222] Next, the measured values of the coordinates of the wafer marksin the sample shots and the design array coordinates of these wafermarks are statistically processed to determine values of EGA parametersincluding a rotation (wafer rotation) Θ₁ of the first-layer shot array,a perpendicularity error W₁ of the first-layer shot array, an offset Ox₁in the direction X2, and an offset Oy₁ in the direction Y2. In thisexample, the wafer 20 was rotated through 90° at the time of exposurefor the first layer, and the X2- and Y2-axes of the second layer areassumed to be perpendicular to each, other. Therefore, the rotation Θ₁is an angle between the Y1-axis of the first-layer shot array and theX2-axis of the second-layer stage coordinate system, and theperpendicularity error W₁ is equal to an angle obtained by subtractingπ/2 (90°) from the angle between the Y1-axis and the axis (−X1-axis)which is in inverse relation to the X1-axis, that is, theperpendicularity error W in FIG. 12(b).

[0223] After the EGA parameters have been obtained as described above,the scanning exposure apparatus 1B, shown in FIG. 11, sets the rotationangle of the wafer 20 such that the Y1-axis of the first-layer shotarray is rotated clockwise relative to the X2-axis through the sameangle as the perpendicularity error W₁ (i.e. W). This means that anoffset of the same angle as the perpendicularity error W₁ is added tothe desired value of the shot array rotation (wafer rotation). As aresult, a straight line 125 which connects together the right-hand edgesof the shot areas 121A to 121C in the first column, which is parallel tothe array direction of the partial shot areas 122A and 122B of the shotarea 121A, for example, becomes parallel to the direction Y2, which isthe scanning direction of the scanning exposure apparatus 1B. In thisstate, a shot array of the second layer is determined by taking intoconsideration the offsets Ox₁ and Oy₁ in the EGA parameters.

[0224]FIG. 14(a) shows a second-layer shot array set over the firstlayer as described above. In FIG. 14(a), for example, second-layer shotareas 126A and 126B are set over the first-layer shot areas 121A to121C; similarly, other second-layer shot areas 126C to 126F are set. Inthis case, the shot area 126A, for example, is divided into threepartial shot areas 127A to 127C in the direction Y2. The partial shotareas 127A to 127C are respectively exposed to images of patterns inpartial pattern areas 113C to 113A of the reticle RB, shown in FIG.14(b). The partial shot areas 127A to 127C in the second-layer shot area126A each has the same size as the size of each of the partial shotareas 122A and 122B in the first-layer shot area 121A, shown in FIG.14(c). The other second-layer shot areas 126B to 126F also each has thesame configuration as that of the second-layer shot area 126A. The shotareas 126A to 126F determined in this way are each exposed to a patternimage of the reticle RB by the scanning exposure method. Thereafter,development and other processing are carried out, thereby allowingpatterns to appear in the second-layer shot areas.

[0225] In this example, as shown in FIG. 14(a), the straight line 125connecting the right-hand edges of the first-layer shot areas 121A to121C is parallel to the direction Y2, which is the scanning directionfor the second layer; therefore, the second-layer shot areas 126A and126B are overlaid on the first-layer shot areas 121A to 121Csubstantially perfectly in both the directions X2 and Y2. Thus, theeffect of the perpendicularity error in the first-layer shot array iseliminated.

[0226] Although in the above-described example the X2- and Y2-axes ofthe stage coordinate system in the scanning exposure apparatus 1B areassumed to be perpendicular to each other as shown in FIG. 13, it shouldbe noted that the X2- and Y2-axes do not necessarily need to beperpendicular to each other. In a case where the X2- and Y2-axes are notperpendicular to each other, the wafer 20 should be rotated such thatthe X1-axis of the first-layer shot array is parallel to the directionY2, which is the scanning direction.

[0227] Next, a second example of the second embodiment of the presentinvention will be described with reference to FIGS. 15 and 16.

[0228] In this example also, the two projection exposure apparatuses(i.e. stepper 1A and scanning exposure apparatus 1B) shown in FIG. 11are used. First, exposure is carried out with respect to the wafer 20 inthe stepper 1A in a state where the reticle RA and the wafer 20 havebeen each rotated through 90° relative to their ordinary positions, asshown in FIGS. 12(a) and 12(b). Next, the wafer 20 is restored to theordinary rotation angle in the scanning exposure apparatus 1B, as shownin FIG. 13, and then, measurement for alignment is carried out. Up tothis point, the second example is approximately the same as the firstexample. In this example, however, the rotation angle of each sampleshot is also measured during the measurement of coordinates of the wafermarks in the sample shots. When the shot area 121A, for example, is asample shot, the rotation angle thereof is measured as follows: Forexample; the coordinates of a pair of wafer marks 123X and 123Y aremeasured, and at the same time, the X2 coordinate of another X-axiswafer mark 124X is measured. A difference between the X2 coordinates ofthe wafer marks 123X and 124X is divided by an approximate value of thedistance in the direction Y2 between the two wafer marks 123X and 124Xto obtain a rotation angle of the shot area 121A. Similarly, rotationangles of the other sample shots are obtained, and a mean value of theobtained rotation angles is defined as a shot rotation θ_(s). Analignment method in which the coordinates of the number of wafer markswhich exceeds two (one in the case of two-dimensional wafer marks) aremeasured in each sample shot is called “in-shot multipoint EGA alignmentmethod”.

[0229] Next, the rotation angle of the wafer 20 is set without adding anoffset to the desired value of the shot array rotation (wafer rotation)in this example.

[0230]FIG. 15 shows the wafer 20 having a rotation angle set asdescribed above on the wafer stage 5B in the scanning exposure apparatus1B, shown in FIG. 11. In FIG. 15, a Y1-axis which indicates one arraydirection of the first-layer shot array is set parallel to the X2-axisof the stage coordinate system in the scanning exposure apparatus 1B. Asa result, the right-hand edges of the shot areas 121A, 121B and 121C,which are parallel to the array direction of the two partial shot areasin the first-layer shot area 121A, for example, are slanted at the sameangle as the perpendicularity error W with respect to the direction Y2,which is the scanning direction. The perpendicularity error W is a valueobtained by subtracting the shot rotation θ_(s), obtained by the in-shotmultipoint EGA method, from the shot array rotation Θ₁ obtained in thestate shown in FIG. 13. If exposure is simply carried out by thescanning exposure method with respect to the wafer 20 which is in thestate shown in FIG. 15, second-layer shot areas would become shot areas126A to 126F having edges parallel to the directions X2 and Y2, as shownby the chain double-dashed lines, resulting in an overlay error betweenthe first- and second-layer shot areas.

[0231] In order to avoid the above problem, in this example, exposure iscarried out with each of the second-layer shot areas 126A to 126Frotated counterclockwise through an angle equal to the perpendicularityerror W relative to the wafer 20. More specifically, in a case where thescanning direction can be finely adjusted during the scanning exposurein the scanning exposure apparatus 1B, the reticle RB is rotatedcounterclockwise through an angle equal to the perpendicularity error Win FIG. 11, and thereafter, the reticle RB is scanned in a directionrotated from the direction Y2 by the perpendicularity error W, and thewafer 20, shown in FIG. 15, is scanned in a direction parallel to thescanning direction of the reticle RB in synchronism with the scanning ofthe reticle RB. To finely adjust the scanning direction of the reticleRB in this way, the reticle RB should be gradually shifted in thedirection X2 according to the scanning position by using, for example, amechanism for finely adjusting the position of the reticle RB. As aresult, the second-layer shot areas 126A to 126F are each rotatedcounterclockwise by the perpendicularity error W, as shown by thecontinuous lines in FIG. 16, and thus the overlay error between thefirst- and second-layer shot areas is minimized.

[0232] It should be noted that, when the exposure apparatus that effectsexposure for the second layer is a one-shot exposure type projectionexposure apparatus (e.g. stepper), the rotation angle (shot rotation) ofeach of the shot areas 126A to 126F can be corrected, as shown in FIG.16, simply by rotating the reticle RB.

[0233] Next, a third example of the second embodiment of the presentinvention will be described with reference to FIGS. 17(a), 17(b) and17(c).

[0234] In this example also, the two projection exposure apparatuses(i.e. stepper 1A and scanning exposure apparatus 1B) shown in FIG. 11are used. First, as shown in FIGS. 12(a) and 12(b), exposure is carriedout with respect to the wafer W in the stepper 1A in a state where thereticle RA and the wafer 20 have been each rotated through 90° relativeto their ordinary positions. Up to this point, the third example is thesame as the first example. In this example, however, the subsequentexposure for the second layer is carried out by the scanning exposureapparatus 1B, shown in FIG. 11, with the reticle RB and the wafer 20left rotated through 90° from their ordinary positions. The scanningexposure apparatus 1B in this example effects scanning exposure in adirection parallel to the direction X2. For this purpose, an exposureapparatus in which the scanning direction is the direction X2 should beused as the scanning exposure apparatus 1B. Alternatively, an exposureapparatus in which the scanning direction can be switched to either ofthe directions X2 and Y2 should be used as the scanning exposureapparatus 1B. The following is a description of the exposure method forthe second layer in this example.

[0235]FIG. 17(a) shows the wafer 20 placed on the wafer stage 5B in thescanning exposure apparatus 1B, shown in FIG. 11, after the completionof exposure and development for the first layer. In FIG. 17(a), theX1-axis which indicates one array direction of first-layer shot areas121A, 121B, . . . , 121I is set approximately parallel to the X2-axis ofthe stage coordinate system in the scanning exposure apparatus 1B. Inthis case, at the time of exposure for the first layer, as shown in FIG.17(c), the reticle RA is set such that the two partial pattern areas112A and 112B lie side-by-side in the direction X1. The first-layer shotarray has a perpendicularity error W.

[0236] In this example also, the wafer 20 shown in FIG. 17(a) issubjected to EGA alignment, thereby obtaining values of EGA parametersincluding a shot array rotation (wafer rotation) Θ₂, a shot arrayperpendicularity error W₂, an offset Ox₂ in the direction X2, and anoffset Oy₂ in the direction Y2. Thereafter, the rotation angle of thewafer 20 is set such that the X1-axis is accurately parallel to theX2-axis on the basis of the rotation Θ₂. Then, as shown in FIG. 17(b),the rotation angle of the reticle RB is set such that the arraydirection of the partial pattern areas 113A to 113C is parallel to thedirection X2. Then, the reticle RB is scanned in the direction +X2 (or−X2) by the scanning exposure apparatus 1B, and the wafer 20 is scannedin the direction −X2 (or +X2) by the scanning exposure apparatus 1B insynchronism with the scanning of the reticle RB, thereby sequentiallytransferring a pattern image of the reticle RB onto the second-layershot areas 126A, 126B, . . . , 126F, shown in FIG. 17(a), by thescanning exposure method. As a result, the second-layer shot areas 126Aand 126B are substantially perfectly overlaid on the first-layer shotareas 121A, 121B and 121C, and thus the effect of the perpendicularityerror W of the first layer is eliminated.

[0237] Although in the above-described second embodiment exposure isfirst carried out by the stepper 1A having a small exposure field, andthereafter, exposure is carried out by the scanning exposure apparatus1B having a large exposure field, it should be noted that the presentinvention is also applicable to an exposure process in which exposure iscarried out by the stepper 1A having a small exposure field afterexposure has been carried out by the scanning exposure apparatus 1Bhaving a large exposure field in reverse relation to the above. In thelatter case also, the effect of the perpendicularity error of the firstlayer can be reduced. Further, although in the above-described secondembodiment the stepper 1A and the scanning exposure apparatus 1B areused as a combination of two exposure apparatuses, it should be notedthat both the two exposure apparatuses may be steppers. Alternatively,both the two exposure apparatuses may be scanning exposure apparatuses.

[0238] According to the second embodiment, when a first mask pattern isto be transferred onto a photosensitive substrate by using the firstexposure apparatus, the array of a plurality of shot areas on thephotosensitive substrate to each of which the first mask pattern is tobe transferred by exposure is set in a direction corresponding to adirection in which the exposure field of the first exposure apparatus isdifferent in length from the exposure field of the second exposureapparatus (i.e. the second exposure field). Therefore, a plurality ofshot areas of a first layer can be arranged in the form of a straightline in the direction in which the exposure fields of the first andsecond exposure apparatuses differ in length from each other.Accordingly, even if the first-layer shot array has a perpendicularityerror, an overlay error between the first and second layers can bereduced by overlaying the second-layer shot areas on the first-layershot areas along the direction in which the two exposure fields aredifferent in length from each other. Thus, it is possible to reduce anoverlay error between different layers in a case where exposure iscarried out by the mix-and-match method using a plurality of exposureapparatuses having respective exposure fields (shot areas) of differentsizes because they are different from each other in the length in apredetermined direction on a photosensitive substrate.

[0239] In a case where the first mask pattern is transferred onto thephotosensitive substrate by using the first exposure apparatus in astate where the photosensitive substrate and the first mask pattern havepreviously been rotated through 90° from their ordinary positions, theeffect of a perpendicularity error of the first-layer shot array can bereadily eliminated without providing a special mechanism on the twoexposure apparatuses, particularly when the first exposure apparatus isa one-shot exposure type exposure apparatus (e.g. stepper).

[0240] In a case where the second exposure apparatus is a scanningexposure type exposure apparatus, and the above-described predetermineddirection (i.e. direction in which the two exposure fields differ inlength from each other) is the scanning direction, the exposure field(i.e. second exposure field) of the second exposure apparatus is likelyto lengthen in the predetermined direction in particular. Accordingly,the present invention is particularly useful in such a case.

[0241] Although in the second embodiment the one-shot exposure typeexposure apparatus is used first and then the scanning exposure typeexposure apparatus is used, these two exposure apparatuses may be usedin the reverse order. That is, the scanning exposure type exposureapparatus may be used first.

[0242] Next, a third embodiment of the exposure method according to thepresent invention will be described with reference to FIGS. 18 to 23(b).In this embodiment, a projection exposure apparatus (stepper) in which areduced image of a pattern formed on a reticle is projected onto eachshot area on a wafer by the step-and-repeat method is used as each oftwo exposure apparatuses. It should be noted that constituent elementsin this embodiment which are similar to those in the first and secondembodiments are denoted by the same reference characters, and thatarrangements similar to those in the first and second embodiments willbe briefly explained in the following description.

[0243]FIG. 18 shows an exposure system used in this embodiment. In theillustrated exposure system are installed a stepper 1A having a smallexposure field (hereinafter referred to as “fine stepper”) and a stepper1B having a large exposure field (hereinafter referred to as “middlestepper”). In this embodiment, the fine stepper 1A is a high-resolutionexposure apparatus, and the middle stepper 1B is a low-resolutionexposure apparatus. The fine stepper 1A is used to carry out exposurefor a critical layer on a wafer, and the middle stepper 1B is used tocarry out exposure for a middle layer on the wafer. However, the finestepper 1A may be a low-resolution exposure apparatus or the middlestepper 1B may be a high-resolution exposure apparatus according to thekind of semiconductor device to be produced.

[0244] First, in the stepper 1A, a pattern area 52A on a reticle RA isilluminated by exposure light from an illumination optical system (notshown), and an image of the original drawing patterns of overlayaccuracy measuring marks (vernier marks), which have been written in thepattern area 52A according to a predetermined layout, is projected ontoa rectangular exposure field 54A on a wafer 20 as an image reduced to ⅕by a projection optical system 3A. A Z1-axis is taken in a directionparallel to an optical axis of the projection optical system 3A, and twoaxes of an orthogonal coordinate system set in a plane perpendicular tothe Z1-axis are defined as an X1-axis and a Y1-axis, respectively. Thereticle RA has an alignment mark 217X for the X1-axis formed at an endof the pattern area 52A in the direction Y1 (e.g. within a maskingframe) and also has an alignment mark 217Y for the Y1-axis formed at anend of the pattern area 52A in the direction X1.

[0245] A wafer stage 5A comprises a Z-stage, an XY-stage, etc. Thecoordinate in the direction X1 of the wafer stage 5A is measured by acombination of a moving mirror 6A and a laser interferometer 7A. Thecoordinate in the direction Y1 of the wafer stage 5A is measured by acombination of a moving mirror 8A and a laser interferometer 9A. Thecoordinates measured by the laser interferometers 7A and 9A are suppliedto a controller 10A which controls operations of the whole apparatus.The controller 10A drives the wafer stage 5A to step, therebypositioning the wafer 20. In this case, the stepping drive of the wafer20 is effected according to a shot map for a critical layer. The shotmap is generated by a map generating unit which comprises a computer inthe controller 10A.

[0246] An off-axis imaging type (FIA type) alignment system 11A imagesan alignment mark (wafer mark) or overlay accuracy measuring verniermark on the wafer 20 and processes an imaging signal thus obtained todetect X1 and Y1 coordinates of the mark. The detected coordinates aresupplied to the controller 10A.

[0247] The middle stepper 1B in this embodiment has substantially thesame arrangement as that of the fine stepper 1A. In the middle stepper1B, however, an image of a pattern formed in a pattern area 52B of areticle RB is projected onto a rectangular exposure field 54B on a wafer20 held on a wafer stage 5B as an image reduced to 1/2.5 through aprojection optical system 3B. Accordingly, the size of the exposurefield 54B is double that of the exposure field 54A of the fine stepper1A in both lengthwise and breadthwise directions. A Z2-axis is taken ina direction parallel to an optical axis of the projection optical system3B, and two axes of an orthogonal coordinate system set in a planeperpendicular to the Z2-axis are defined as an X2-axis and a Y2-axis,respectively. The pattern area 52B of the reticle RB is divided into twocolumns in the direction X2 and two rows in the direction Y2 to formpartial pattern areas 218A to 218D. The partial pattern areas 218A to218D each has vernier mark original drawing patterns formed according tothe same layout.

[0248] The X2 coordinate of the wafer stage 5B in the middle stepper 1Bis measured by a combination of a moving mirror 6B and a laserinterferometer 7B. The Y2 coordinate of the wafer stage 5B is measuredby a combination of a moving mirror 8B and a laser interferometer 9B.The measured coordinates of the wafer stage 5B are supplied to acontroller 10B. The controller 10B controls stepping of the wafer stage5B. Stepping drive of the wafer stage 5B is effected according to anarray of shot areas (to each of which the pattern image of the patternarea 52B is to be projected by exposure) set on an exposure surface ofthe wafer 20, which is to be exposed, that is, a shot map for a middlelayer. The shot map is generated by a map generating unit whichcomprises a computer in the controller 10B.

[0249] In this case, the map generating unit in the controller 10A andthe map generating unit in the controller 10B have the function ofsupplying shot map information prepared thereby to each other. Whenexposure for a middle layer is to be carried out over a critical layer,for example, shot map information for the critical layer prepared by themap generating unit in the controller 10A of the stepper 1A istransmitted from a communication unit in the controller 10A to acommunication unit in the controller 10B. The map generating unit in thecontroller 10B generates a shot map for the middle layer on the basis ofthe supplied shot map information. Conversely, when exposure for acritical layer is to be carried out over a middle layer, shot mapinformation for the middle layer prepared by the map generating unit inthe controller 10B is supplied to the map generating unit in thecontroller 10A.

[0250] The middle stepper 1B also has an off-axis imaging type (FIAtype) alignment system 11B provided at a side surface of the projectionoptical system 3B. The alignment system 11B detects X2 and Y2coordinates of a wafer mark or vernier mark on the wafer 20.

[0251] Next, one example of an operation of correcting coordinatetransformation parameters for alignment when exposure of the pattern forthe middle layer is to be effected by the middle stepper 1B afterexposure of the pattern for the critical layer has been carried out bythe fine stepper 1A in this embodiment will be explained for each of thefirst to third processing steps. In this embodiment also, the EGA(Enhanced Global Alignment) method is used in which values of the sixcoordinate transformation parameters (scaling parameters Rx and Ry,rotation Θ, perpendicularity W, and offsets Ox and Oy) in Eq. (1) aredetermined, and array coordinates of each shot area are calculated fromthe coordinate transformation parameters and design array coordinates.

[0252] First, the first step will be explained.

[0253] In the first step, an unexposed wafer 20 coated with aphotoresist is placed on the wafer stage 5A of the fine stepper 1A,shown in FIG. 18, and a reduced image of the pattern on the reticle RAis sequentially transferred by the step-and-repeat method onto amultiplicity of shot areas arrayed on the wafer 20 in units of theexposure field 54A. The reticle RA has the original drawing patterns ofa plurality of vernier marks formed according to a predetermined layoutin addition to a pair of alignment marks. Thereafter, the wafer 20 issubjected to development, thereby allowing the pair of alignment marksto appear as wafer marks comprising recess-and-projection patterns, andalso allowing the vernier mark original drawing patterns to appear asvernier marks comprising recess-and-projection patterns. The patternsobtained after the development can be regarded as critical layerpatterns on the wafer 20. However, it is also possible to carry out thefollowing alignment and measurement of an amount of positionaldisplacement between two corresponding vernier marks with these marksleft in the form of latent images without effecting development.

[0254] Next, the second step will be explained.

[0255] A photoresist is coated over the wafer 20 having the wafer andvernier marks formed in the first step, and the photoresist-coated wafer20 is placed on the wafer stage 5B of the middle stepper 1B, shown inFIG. 18. At this time, information concerning the critical layer shotmap used in the first step has been supplied from the controller 10A ofthe fine stepper 1A to the controller 10B of the middle stepper 1B.Thus, the controller 10B can obtain design array coordinates of thecritical layer wafer marks on the wafer 20.

[0256]FIG. 19(a) shows the wafer 20 placed on the wafer stage 5B. InFIG. 19(a), the X2- and Y2-axes of the middle stepper 1B are shown asbeing X- and Y-axes, respectively. In this case, the wafer 20 has beenroughly aligned by a pre-alignment mechanism (not shown), and thesurface of the wafer 20 has been divided into M (M is an integer of 12or more) critical layer shot areas SE1, SE2, . . . , SEM in twodirections which are approximately parallel to the directions X and Y,respectively. In actual practice, a scribe line area of a predeterminedwidth lies between shot areas SEm (m=1 to M); however, illustration ofthe scribe line area is omitted in FIG. 19(a). The width (pitch) in thedirection x of each shot area SEm, including the scribe line area, is d,and the width (pitch) in the direction Y is c. In this embodiment, eachshot area SEm is approximately square (d≈c).

[0257]FIG. 19(b) shows a shot area SEm as a typical example of thecritical layer shot areas. In FIG. 19(b), the shot area SEm has a wafermark 221X for the X-axis formed at one end thereof, and also has a wafermark 221Y for the Y-axis formed at another end thereof. Further, theshot area SEm has five vernier marks 222A to 222E which are distributedin a cross shape, and also has four vernier marks 223A to 223D which areformed at respective positions near the four corners of the shot areaSEm. The original drawing patterns of marks distributed as shown in FIG.19(b) have been formed in the pattern area 52A of the reticle RA of thefine stepper 1A, shown in FIG. 18.

[0258] It should be noted that the vernier marks 222A to 222E and 223Ato 223D used in this embodiment are two-dimensional box-in-box marks,which are detected by an imaging detection method with the alignmentsystem 11B, shown in FIG. 18. However, it is possible to use other kindsof mark as vernier marks, for example, marks each formed by acombination of two one-dimensional line-and-space patterns which arecrossed at right angles. It is also possible to use the wafer marks 221Xand 221Y as vernier marks. Further, marks which are detected, forexample, by the laser step alignment (LSA) method may also be used asvernier marks. The distribution of vernier marks is not necessarilylimited to that shown in FIG. 19(b). That is, vernier marks used in thisembodiment may be distributed as desired.

[0259] Next, the controller 10B of the middle stepper 1B, shown in FIG.18, effects alignment by the EGA method. Accordingly, the controller 10Bdrives the wafer stage 5B to move the field of view of the alignmentsystem 11B sequentially according to the critical layer shot map,thereby measuring array coordinates (Mxn,Myn) in a stage coordinatesystem (i.e. a coordinate system determined by values measured with thelaser interferometers 7B and 9B of the middle stepper 1B) of each of thewafer marks 221X and 221Y attached to 10 (for example) shot areas(sample shots) SEa, SEb, SEc, . . . , SEj selected from among the shotareas on the wafer 20, as shown in FIG. 19(a). Then, values of the sixEGA parameters (scaling parameters Rx and Ry, rotation Θ,perpendicularity W, and offsets Ox and Oy) in Eq. (1) are determined soas to minimize the residual error component (expressed by Eq. (2)) ofthe alignment error, that is, the deviation of the measured values(Mxn,Myn) of the wafer marks 221X and 221Y of each sample shot from thearray coordinate values, which are calculated from the design arraycoordinates (Dxn,Dyn) of the wafer marks 221X and 221Y.

[0260] Next, the controller 10B sequentially substitutes the six EGAparameters and the design array coordinate values (Dxm,Dym) of the shotareas SEm (m=1 to M) into the right-hand side of Eq. (1), therebyobtaining array coordinate values in the stage coordinate system of eachshot area Sem of the critical layer on the wafer 20. At this time,because the exposure field 54B of the middle stepper 1B is twice aslarge as the exposure field 54A of the fine stepper 1A in both thedirections X and Y, the controller 10B divides the shot areas SEm (m=1to M), shown in FIG. 19(a), into a plurality of blocks each comprisingan array of two shot areas in the direction X and two shot areas in thedirection Y, and obtains array coordinates in the stage coordinatesystem of the center of each block from the computational arraycoordinates of the four shot areas in the block. Thereafter, thecontroller 10B sequentially aligns the array coordinates of the centerof each block on the wafer 20 with the center of the exposure field 54B,and transfers an image of the vernier mark original drawing patternsformed on the reticle RB onto each block. Thereafter, development iscarried out, thereby allowing middle layer vernier marks to appear overthe critical layer vernier marks on the wafer 20. It should be notedthat the following measurement may be effected with the transferredmarks left in the form of latent images, as has already been describedabove.

[0261] Next, a third step will be explained.

[0262] In the third step, an amount of positional displacement betweenthe critical and middle layer vernier marks is measured. For thispurpose, the wafer 20 having been subjected to development in the secondstep is placed, for example, on the wafer stage 5B of the middle stepper1B, shown in FIG. 18, and an amount of positional displacement betweenthe critical and middle layer vernier marks is measured by the alignmentsystem 11B. However, the measurement of a positional displacementbetween the critical and middle layer vernier marks may be carried outby using another measuring device of high accuracy.

[0263]FIG. 20(a) shows the wafer 20 having overlaid vernier marks formedby the exposure process in the second step. In FIG. 20(a), the wafer 20has middle layer shot areas SF1, SF2, . . . , SFN (N is an integer of 4or more) arranged at a pitch 2 d along the X-axis and at a pitch 2 calong the Y-axis. Each shot area SFn (n=1 to N) contains four criticallayer shot areas SEm. It should be noted that, if there is amagnification error in each shot area SFn of the middle layer, thewidths of each shot area SFn in the directions X and Y are slightlydeviated from 2 d and 2 c, respectively. Further, the center 261 of eachshot area SFn is substantially coincident with the center of theassociated four critical layer shot areas. Each shot area SFn has atotal of 36 (=4×9) vernier marks corresponding to the nine vernier marks222A to 222E and 223A to 223D (see FIG. 19(b)) in each critical layershot area SEm.

[0264] Assuming that each middle layer shot area SFn is M₁/N₁ times andM₂/N₂ times as large as the critical layer shot area SEm in thedirections X and Y, respectively, in this embodiment M₁/N₁=2/1 andM₂/N₂=2/1. Accordingly, a reference measurement area in this embodimentis an area determined by multiplying the middle layer shot area SFn byone in each of the directions X and Y, that is, the shot area SFnitself. Therefore, four shot areas (reference measurement areas) SFa toSFd which are substantially uniformly distributed over the wafer 20, asshown by the hatching in the figure, are defined as objects to bemeasured.

[0265]FIG. 20(b) shows the shot area SFa among the four referencemeasurement areas. In FIG. 20(b), the shot area SFa has nine middlelayer vernier marks 224A to 224E and 225A to 225D formed to surround therespective vernier marks which belong to the second-quadrant shot areaSEp of the four critical layer shot areas underlying the middle layershot area SFa. The shot area SFa further has nine vernier marks (notshown) similarly formed to surround the respective vernier marks whichbelong to each of the other shot areas SE(p+1), SEq and SE(q+1)underlying the shot area SFa. However, FIG. 20(b) shows the middle layervernier mark 226C corresponding to the vernier mark 222C formed in theintermediate portion at the right end of the first-quadrant shot areaSE(p+1) among those middle layer vernier marks.

[0266] Next, in this embodiment, an amount of positional displacementbetween the critical layer vernier mark 222C and the middle layervernier mark 226C is measured at each of measuring points 232A to 232Dlying at the mutually identical positions in the shot areas (referencemeasurement areas) SFa to SFd, which are objects to be measured, on thewafer 20. For example, the measuring points 232A to 232D each lies inthe intermediate portion at the right end of the first quadrant [i.e. anarea corresponding to the shot area SE(p+1) in FIG. 20(b)] in the shotareas SFa to SFd. Accordingly, at the measuring point 232A, positionaldisplacements (Δxa,Δya) in the directions X and Y of the vernier mark226C relative to the vernier mark 222C is measured. At the othermeasuring points 232B to 232D, positional displacements (Δxb,Δyb) to(Δxd,Δyd) are similarly measured.

[0267] Thereafter, if there is a difference (Δxb−Δxd) between thepositional displacements in the direction X measured at the twomeasuring points 232D and 232B in FIG. 20(a), for example, thedifference (Δxb−Δxd) is divided by the distance in the direction Xbetween the two measuring points 232D and 232B, thereby obtaining acorrection value (error) ΔRx for the scaling parameter Rx in thedirection X among the EGA parameters. If there is a difference (Δyb−Δyd)between the positional displacements in the direction Y measured at themeasuring points 232D and 232B, the difference (Δyb−Δyd) is divided bythe distance in the direction X between the two measuring points,thereby obtaining a correction value ΔΘ for the rotation Θ among the EGAparameters. Further, mean values of the positional displacements in thedirections X and Y measured at the four measuring points are defined ascorrection values ΔOx and ΔOy for the offsets Ox and Oy among the EGAparameters. Similarly, correction values ΔRy and ΔW for the other EGAparameters, that is, the scaling parameter Ry and the perpendicularityW, are also obtained. These correction values are stored in a storageunit in the controller 10B of the middle stepper 1B. It should be notedthat, if positional displacements between the corresponding verniermarks are measured with another measuring device, and correction valuesare obtained by using another computer or the like, the operator inputsthe correction values to the controller 10B through an input device.Thus, the third step is terminated.

[0268] Thereafter, in a case where exposure is carried out by themix-and-match method using the fine stepper 1A and the middle stepper1B, shown in FIG. 18, a critical layer pattern is formed on the wafer 20by using the fine stepper 1A, and before a middle layer pattern isformed by using the middle stepper 1B, coordinate values ofpredetermined sample shots in the stage coordinate system are measured,and values of the six EGA parameters in Eq. (1) are determined on thebasis of the measured coordinate values. Thereafter, the controller 10Badds the EGA parameter correction values (ΔRx, ΔRy, ΔΘ, ΔW, ΔOx, andΔOy), stored in the above-described third step, to the determined EGAparameters (Rx, Ry, Θ, w, Ox, and Oy) to obtain corrected EGAparameters. Then, the controller 10B calculates coordinate positions ofthe shot areas of the critical layer by using the corrected EGAparameters, calculates exposure positions of the shot areas of themiddle layer on the basis of the coordinate positions of the criticallayer shot areas, and sequentially transfers the reticle pattern for themiddle layer onto the middle layer shot areas on the basis of theexposure positions.

[0269] In this embodiment, the measuring points used in theabove-described third step are at the mutually identical positions inthe shot areas (reference measurement areas) SFa to SFd, as shown inFIG. 20(a). Accordingly, even when the middle layer shot areas SFn havea magnification error or a rotation error, the same offset value issuperimposed at each measuring point, and the magnification or rotationerror affects only the offsets Ox and Oy among the EGA parameters. Thus,the overlay accuracy between the critical and middle layers improvesbecause the values of other influential linear parameters (Rx, Ry, Θ,and W) are accurate.

[0270] In the above-described embodiment, a magnification error orrotation error in the middle layer shot areas affects the offsets Ox andOy among the EGA parameters. Therefore, the effect of the magnificationor rotation error is eliminated by averaging process. The method ofeliminating the magnification or rotation error will be explained belowwith reference to FIGS. 21(a) to 23(b), in which portions correspondingto those in FIGS. 20(a) and 20(b) are denoted by the same referencecharacters.

[0271]FIG. 21(a) shows a first method of arranging measuring points foreliminating an offset error. In FIG. 21(a), four middle layer shot areasSFa to SFd are selected as reference measurement areas on the wafer 20in the same way as in FIG. 20(a). Then, an amount of positionaldisplacement between two corresponding vernier marks is measured at eachof four measuring points in the shot area SFa, that is, a measuringpoint 233A at the bottom left in the first quadrant, a measuring point235A at the bottom right in the second quadrant, a measuring point 234Aat the top right in the third quadrant, and a measuring point 236A atthe top left in the fourth quadrant. Mean values of the positionaldisplacements in the directions X and Y measured at the four measuringpoints are assumed to be (Δxa′,Δya′).

[0272] More specifically, FIG. 21(b) is an enlarged view of the shotarea SFa. As shown in FIG. 21(b), an amount of positional displacementbetween the vernier mark 223D in the shot area SE(p+1) and the middlelayer vernier mark 227D is measured at the measuring point 233A, and anamount of positional displacement between the vernier mark 223C in theshot area SEp and the middle layer vernier mark 225C is measured at themeasuring point 235A. Similarly, an amount of positional displacementbetween the vernier mark 223B (or 223A) and the vernier mark 229B (or231A) is measured at the measuring point 234A (or 236A).

[0273] Referring to FIG. 21(a), positional displacement is similarlymeasured in each of the other shot areas SFb to SFd. That is, in eachshot area, an amount of positional displacement between the twocorresponding vernier marks is similarly measured at each of the fourmeasuring points lying at respective positions mutually identical withthe measuring points 233A to 236A in the shot area SFa, and mean valuesof the measured amounts of positional displacement are determined to be(Δxb′,Δyb′) to (Δxd′,Δyd′). Thereafter, EGA parameter correction valuesare obtained from the amounts of positional displacement measured in thefour shot areas SFa to SFd. In this case, even if the shot area SFa, forexample, has a magnification error or rotation error (shot rotationerror), the effect of such an error appears symmetrically at the fourmeasuring points 233A to 236A; therefore, the effect of themagnification or rotation error can be eliminated by averaging theamounts of positional displacement at the four measuring points.Accordingly, even if there is a magnification or rotation error, noerror will be introduced into the offsets Ox and Oy in the EGAparameters.

[0274] Further, the exposure method according to this embodimentprovides the following advantageous effects: Since the measuring points233A to 236A lie in the center of the shot area SFa, the distortionintroduced by the projection optical system 3B of the middle stepper 1Bis small at the measuring points 233A to 236A, and thus the distortionof the middle layer shot areas produces a minimal effect on themeasurement result. Further, in the shot area SFa, for example,measurement is carried out in each of four different corners of the fourcritical layer shot areas. Therefore, the effects of the magnificationor rotation errors in the critical layer shot areas are canceled by theaveraging process. Similarly, the effect of the distortion of thecritical layer shot areas is reduced by the averaging process.

[0275] In this embodiment, it is only necessary to enable measuringpoints to be symmetrically disposed in each middle layer shot area usedas a reference measurement area. Therefore, in FIG. 21(a), only twomeasuring points shown by the black circles may be selected from each ofthe shot areas SFa to SFd, for example, (i.e. the measuring points 233Aand 234A in the shot area SFa). Alternatively, only two measuring pointsshown by the white circles may be selected from each of the shot areasSFa to SFd (i.e. the measuring points 235A and 236A in the shot areaSFa).

[0276] Although in the above-described embodiment the measuring pointsare concentrated on the center of each of the middle layer shot areasused as reference measurement areas, the arrangement of measuring pointsis not necessarily limited to it. As shown in FIG. 22(a), measuringpoints 237A to 240A may be set in the respective centers of the fourcritical layer shot areas in the shot area SFa, for example. It is alsopossible to select two measuring points 237A and 238A, shown by theblack circles, or two measuring points 239A and 240A, shown by the whitecircles, from among the four measuring points. In this case, thedistortion of the critical layer shot areas is minimized, and the effectof the distortion of the middle layer shot areas is reduced by theaveraging process.

[0277] However, in a case where the distortion of the critical andmiddle layer shot areas has previously been known to be small, as shownfor example in FIG. 22(b), four measuring points 241A to 244A in thefour corners of the shot area SFa may be selected. Alternatively, onlytwo measuring points 241A and 242A, shown by the black circles, or onlytwo measuring points 243A and 244A, shown by the white circles, may beselected.

[0278] To sum up, an efficient measuring point layout which enables theaveraging effect to be obtained and which makes it possible to minimizethe number of measuring points and to shorten the time required formeasurement is such as that shown, for example, in FIG. 23(a) or 23(b).In the layout shown in FIG. 23(a), measuring points 233A to 233D and234A to 234D are selected in four shot areas (reference measurementareas) SFa to SFd on the wafer 20. More specifically, the measuringpoints 233A to 233D are each on the right side of the center of theassociated shot area, toward the top as viewed in the figure, and themeasuring points 234A to 234D are each on the left side of the center ofthe associated shot area, toward the bottom as viewed in the figure. Inthe layout shown in FIG. 23(b), measuring points are selected asfollows: In a pair of mutually opposing shot areas SFa and SFc among thefour shot areas, measuring points 235A and 235C are selected which areeach on the left side of the center of the associated shot area, towardthe top, and measuring points 236A and 236C are selected which are eachon the right side of the center of the associated shot area, toward thebottom; in the other pair of mutually opposing shot areas SFb and SFd,measuring points 233B and 233D are selected which are each on the rightside of the center of the associated shot area, toward the top, andmeasuring points 234B and 234D are selected which are each on the leftside of the center of the associated shot area, toward the bottom.

[0279] It is also possible to select from each reference measurementarea one measuring point which is at a symmetric position with respectto the center of the area, as shown in FIGS. 24(a) and 24(b). That is,FIG. 24(a) shows middle layer shot areas transferred over a criticallayer on the wafer 20. From among the shot areas, eight shot areas SFato SFh, which are substantially uniformly distributed, are selected asreference measurement areas. The shot areas SFa to SFh each containsfour critical layer shot areas.

[0280] Then, from the two shot areas SFc and SFg, measuring points 233Cand 233G are respectively selected which are each on the right side ofthe center of associated shot area, toward the top, and from the twoshot areas SFa and SFe, measuring points 235A and 235E are respectivelyselected which are each on the left side of the center of the associatedshot area, toward the top. From the two shot areas SFd and SFh,measuring points 234C and 234H are respectively selected which are eachon the left side of the center of the associated shot area, toward thebottom, and from the other two shot areas SFb and SFf, measuring points236B and 236F are respectively selected which are each on the right sideof the center of the associated shot area, toward the bottom. Then, anamount of positional displacement between the critical and middle layervernier marks is measured at each of the selected measuring points. Inthis example, thereafter, the amounts of positional displacementmeasured at two measuring points which are at symmetric positions in apair of middle layer shot areas (e.g. the measuring points 235A and236B) are averaged, thereby reducing the effect of the magnification orrotation error of the middle layer. The effects of the middle layerdistortion, the reticle writing error, etc. are also reduced by theaveraging process.

[0281] Let us consider the measuring method in this example in terms ofone critical layer shot area SE, as shown in FIG. 24(b). In thisexample, measurement is carried out twice at each of measuring points245A to 245D in the four corners of the shot area SE. Therefore,assuming that the magnification or rotation error of the critical layershot areas is substantially uniform over the wafer, it is possible toreduce the effects of magnification error, rotation error, anddistortion of the critical layer shot areas, reticle writing error, etc.by averaging the amounts of positional displacement measured, forexample, at a pair of mutually opposing measuring points (e.g. themeasuring points 245A and 245C) among the measuring points 245A to 245Din the four corners of the shot area SE.

[0282] As has been described above, the third embodiment shows ameasuring point layout which is applicable in a case where the size ofeach middle layer shot area is twice as large as each critical layershot area in each of the directions X and Y, and where one chip pattern,for example, is formed in each critical layer shot area. In actuality,however, two or more chip patterns may be contained in each criticallayer shot area; there are various size ratios of the middle layer shotareas to the critical layer shot areas. Further, projection exposureapparatuses usable in the third embodiment are not necessarily limitedto one-shot exposure type projection exposure apparatuses such assteppers; it is also possible to use scanning exposure type projectionexposure apparatuses, e.g. step-and-scan type projection exposureapparatuses in which a pattern on a reticle is sequentially transferredonto each shot area on a wafer by synchronously scanning the reticle andthe wafer with respect to a projection optical system. Various othermodifications of the third embodiment will be explained below withreference to FIGS. 25(a) to 27(b).

[0283] In the modification shown in FIGS. 25(a) to 25(c), each criticallayer shot area SE has, as shown in FIG. 25(a), two identical chippatterns 246A and 246B arranged in the direction Y. As shown in FIG.25(b), each middle layer shot area SF has identical chip patternsarranged in two columns in the direction X and four rows in thedirection Y. In this case, assuming that each chip pattern is arectangular pattern having a width b in the direction X and a width a inthe direction Y, the width in the direction X of the critical layer shotarea SE is b, and the width in the direction Y of the shot area SE is 2a. The width in the direction X of the middle layer shot area SF is 2 b,and the width in the direction Y of the shot area SF is 4 a.Accordingly, the shot area SF is 2/1 times as large as the shot area SEin each of the directions X and Y. Therefore, as shown in FIG. 25(c), areference measurement area SG, which has a size regarded as being theleast common multiple of the sizes of the shot areas SE and SF, has awidth 2 b in the direction X and a width 4 a in the direction Y. Thatis, the reference measurement area SG has the same size as that of themiddle layer shot area SF. Accordingly, when a measuring point 247, forexample, is selected in a certain reference measurement area SG, in theother reference measurement areas also measuring points which are at theidentical positions with the measuring point 247 are selected. By doingso, EGA parameter correction values can be accurately obtained.

[0284] However, in order to reduce the effect of the magnificationerror, rotation error, etc. of the middle layer shot areas, it isdesirable to select, for example, measuring points which are insymmetric relation to the measuring point 247 with respect to the centerposition in the reference measurement areas in the same way as in theabove-described third embodiment. The same is true of the followingmodifications.

[0285] In the modification shown in FIGS. 26(a) to 26(c), a criticallayer shot area SE has, as shown in FIG. 26(a), two identical chippatterns arranged in the direction Y. As shown in FIG. 26(b), a middlelayer shot area SH has three identical chip patterns arranged in thedirection Y. Further, the middle layer projection exposure apparatus isof the scanning exposure type. Thus, the shot area SH is exposed byscanning the wafer with respect to a slit-shaped exposure area 248.

[0286] At this time, assuming that the critical layer shot area SE has awidth b in the direction X and a width 2 a in the direction Y, the widthin the direction X of the middle layer shot area SH is b, and the widthin the direction Y of the shot area SH is 3 a. Accordingly, the shotarea SH is 1/1 time as large as the shot area SE in the direction X, andthe former is 3/2 times as large as the latter in the direction Y.Therefore, as shown in FIG. 26(c), a reference measurement area SI,which has a size regarded as being the least common multiple of thesizes of the shot areas SE and SH, has a width b in the direction X anda width 6 a in the direction Y. In this modification also, when ameasuring point 249, for example, is selected in a certain referencemeasurement area SI, in the other reference measurement areas alsomeasuring points which are at the identical positions with the measuringpoint 249 are selected. By doing so, EGA parameter correction values canbe accurately obtained.

[0287] In this regard, FIG. 27(a) shows an enlarged view of one exampleof the reference measurement area SI, shown in FIG. 26(c). In FIG.27(a), a pair of adjacent shot areas SH1 and SH2 exposed by the scanningexposure method contain three critical layer shot areas SEI, SE2 andSE3. FIG. 27(b) shows an expansion and contraction quantity ΔY in thelongitudinal direction (direction Y) in the shot areas SHI and SH2,shown in FIG. 27(a), due to a magnification error. The expansion andcontraction quantity ΔY in the direction Y changes at a period which isequal to the length of each of the shot areas SH1 and SH2. Accordingly,if the centers of the critical layer shot areas SE1 to SE3 are definedas measuring points 250A to 250C, for example, the expansion andcontraction quantities of the middle layer shot areas measured at themeasuring points 250A to 250C show different values as shown by thepositions 251A to 251C in FIG. 27(b). Accordingly, if a given measuringpoint 249 is selected in a certain reference measurement area SI in FIG.26(c), the measurement result is affected by the magnification error ofthe middle layer shot areas unless measuring points are selected at theidentical positions with the measuring point 249 in the other referencemeasurement areas.

[0288] In the modification shown in FIGS. 28(a) to 28(c), a criticallayer shot area SF has, as shown in FIG. 28(a), identical chip patternsarranged in three rows in the direction Y and two columns in thedirection X. As shown in FIG. 28(b), a middle layer shot area SH exposedby the scanning exposure method has three identical chip patternsarranged in the direction Y. In this case, assuming that the width inthe direction X of the critical layer shot area SF is 2 b, and the widthin the direction Y of the shot area SF is 3 a, the width in thedirection X of the middle layer shot area SH is b, and the width in thedirection Y of the shot area SH is 3 a. Accordingly, as shown in FIG.28(c), a reference measurement area SJ, which has a size regarded asbeing the least common multiple of the sizes of the shot areas SF andSH, has a width 2 b in the direction X and a width 3 a in the directionY. That is, the reference measurement area SJ has the same size as thatof the critical layer shot area SF. In this modification also, when ameasuring point 252, for example, is selected in a certain referencemeasurement area SJ, in the other reference measurement areas alsomeasuring points which are at the identical positions with the measuringpoint 252 are selected. By doing so, EGA parameter correction values canbe accurately obtained.

[0289] Although in the above-described third embodiment andmodifications thereof a combination of two steppers or a combination ofa stepper and a step-and-scan type projection exposure apparatus isused, it should be noted that a combination of usable projectionexposure apparatuses is not necessarily limited to the above. Forexample, it is also possible to use two different step-and-scan typeprojection exposure apparatuses as an exposure apparatus having a smallexposure field and an exposure apparatus having a large exposure field.

[0290] According to the exposure method in the third embodiment, an areawhich is so large as to contain an integer number of first and secondexposure fields in each of two directions (i.e. an area having a sizeregarded as being the least common multiple of the sizes of the firstand second exposure fields) is defined as a reference measurement area,and an amount of positional displacement between two correspondingoverlay accuracy measuring marks (i.e. vernier marks) is measured ateach of measuring points lying at the mutually identical positions in apredetermined number of reference measurement areas. Therefore, there isno possibility that the effect of a magnification or rotation error, forexample, of the second mask pattern will appear as a linear expansionand contraction error or rotation error in alignment errors which mayarise during the exposure of the second mask pattern. Accordingly, it ispossible to increase the overlay accuracy between a critical layerpattern and a middle layer pattern in a case where exposure is carriedout by the mix-and-match method with respect to a substrate where acritical layer and a middle layer are mixedly present.

[0291] In a case where the second exposure apparatus calculates anexposure position by using coordinate transformation parameters andobtains correction values for the parameters from results of measurementcarried out for each reference measurement area, the overlay accuracycan be increased because a magnification or rotation error of the secondmask pattern has no effect on parameters indicating linear expansion andcontraction, rotation and perpendicularity among the coordinatetransformation parameters.

[0292] Regarding offset parameters, the effect of a magnification orrotation error of the second mask pattern can be reduced, for example,by using mean values of results of measurement carried out at measuringpoints disposed symmetrically with respect to the center point in thereference measurement areas.

[0293] Next, one example of a fourth embodiment of the exposure methodaccording to the present invention will be described with reference toFIGS. 29 to 32. Two exposure apparatuses used in this example are aone-shot exposure type projection exposure apparatus (stepper) with ademagnification ratio of 5:1 and a step-and-scan type projectionexposure apparatus with a demagnification ratio of 4:1. In this example,two chip patterns are formed in each shot area exposed by the formerprojection exposure apparatus (i.e. a two-chip reticle is used), andthree chip patterns are formed in each shot area scan-exposed by thelatter projection exposure apparatus (i.e. a three-chip reticle isused). It should be noted that constituent elements in this examplewhich are similar to those in the first to third embodiments are denotedby the same reference characters and will be briefly explained below.

[0294]FIG. 29 shows an exposure system used in this embodiment. In FIG.29, a one-shot exposure type projection exposure apparatus (hereinafterreferred to as “fine stepper”) 1A, and a step-and-scan type projectionexposure apparatus (hereinafter referred to as “scanning exposureapparatus”) 1B are installed. In this embodiment, the fine stepper 1A isa high-resolution exposure apparatus, while the scanning exposureapparatus 1B is a low-resolution exposure apparatus. The fine stepper 1Ais used to carry out exposure for a critical layer on a wafer, and thescanning exposure apparatus 1B is used to carry out exposure for amiddle layer on the wafer.

[0295] First, in the fine stepper 1A, a pattern area 62A on a reticle RAis illuminated by exposure light from an illumination optical system(not shown), and an image of a pattern formed in the pattern area 62A isformed on a rectangular exposure field 64A on a wafer 20 as a projectedimage reduced to ⅕ by a projection optical system 3A. A Z1-axis is takenin a direction parallel to an optical axis of the projection opticalsystem 3A, and two axes of an orthogonal coordinate system set in aplane perpendicular to the Z1-axis are defined as an X1-axis and aY1-axis, respectively. The pattern area 62A on the reticle RA is dividedinto partial pattern areas 312A and 312B of the same size in thedirection Y1. The partial pattern areas 312A and 312B each has originaldrawing patterns of alignment marks and overlay accuracy measuring marks(vernier marks) written according to the same layout.

[0296] A wafer stage 5A comprises a Z-stage, an XY-stage, etc. Thecoordinate in the direction X1 of the wafer stage 5A is measured by acombination of a moving mirror 6A and a laser interferometer 7A. Thecoordinate in the direction Y1 of the wafer stage 5A is measured by acombination of a moving mirror 8A and a laser interferometer 9A. Thecoordinates measured by the laser interferometers 7A and 9A are suppliedto a controller 10A which controls operations of the whole apparatus.The controller 10A drives the wafer stage 5A to step, therebypositioning the wafer 20. The stepping drive of the wafer 20 is effectedaccording to a shot map for a critical layer. The shot map is generatedby a map generating unit which comprises a computer in the controller10A.

[0297] An off-axis imaging type (FIA type) alignment system 11A imagesan alignment mark (wafer mark) on the wafer 20 to detect X1 and Y1coordinates of the mark. The detected coordinates are supplied to thecontroller 10A.

[0298] Next, in the scanning exposure apparatus 1B in this example, apart of a pattern area 62B on a reticle RB is illuminated by exposurelight from an illumination optical system (not shown), and an image of apart of the reticle pattern is formed in a slit-shaped exposure area 314on a wafer 20, which is held on a wafer stage 5B, as a projected imagereduced to ¼ by a projection optical system 3B. The reticle RB isscanned in the direction −Y2 (or +Y2), and the wafer 20 is scanned inthe direction +Y2 (or −Y2) in synchronism with the scanning of thereticle RB, thereby sequentially projecting an image of the patternformed in the pattern area 62B of the reticle RB onto the exposure field64B on the wafer 20.

[0299] The pattern area 62B of the reticle RB is divided into threepartial pattern areas 313A to 313C of the same size in the direction Y2,which is the scanning direction. The size of the exposure field 64B issuch that its dimension in the scanning direction is 3/2 times thedimension of the exposure field 64A of the fine stepper 1A, and theexposure field 64B is equal in size (1:1) to the exposure field 64A inthe non-scanning direction. The partial pattern areas 313A to 313C alsoeach has original drawing patterns of vernier marks formed according tothe same layout.

[0300] The position of a reticle stage (not shown) for scanning thereticle RB of the scanning exposure apparatus 1B and the X2 and Y2coordinates of the wafer stage 5B are supplied to a controller 10B. Thecontroller 10B controls synchronous drive of the reticle stage (notshown) and the wafer stage 5B. The scanning exposure operation of thewafer stage 5B is effected according to a shot map for a middle layerset on an exposure surface of the wafer 20, which is to be exposed. Theshot map is generated by a map generating unit which comprises acomputer in the controller 10B.

[0301] In this case, the map generating unit in the controller 10A andthe map generating unit in the controller 10B have the function ofsupplying shot map information prepared thereby to each other.

[0302] The scanning exposure apparatus 1B also has an off-axis imagingtype (FIA type) alignment system 11B provided at a side surface of theprojection optical system 3B. The alignment system 11B detects X2 and Y2coordinates of a wafer mark or vernier mark on the wafer 20.

[0303] Next, one example of an operation of correcting in-shotparameters (i.e. shot magnifications rx and ry, shot rotation θ, andshot perpendicularity w) when exposure of the pattern for the middlelayer is to be effected by the scanning exposure apparatus 1B afterexposure of the pattern for the critical layer has been carried out bythe fine stepper 1A in this example will be explained for each of thefirst to third processing steps.

[0304] First, the first step will be explained.

[0305] In the first step, an unexposed wafer 20 coated with aphotoresist is placed on the wafer stage 5A of the fine stepper 1A,shown in FIG. 29, and a reduced image of the pattern on the reticle RAis sequentially transferred by the step-and-repeat method onto amultiplicity of shot areas arrayed on the wafer 20 in units of theexposure field 64A. The reticle RA has original drawing patterns of twosets of vernier marks formed according to a predetermined layout inaddition to two pairs of alignment marks. Thereafter, the wafer 20 issubjected to development, thereby allowing the two pairs of alignmentmarks to appear as wafer marks comprising recess-and-projectionpatterns, and also allowing the two sets of vernier mark originaldrawing patterns to appear as vernier marks comprisingrecess-and-projection patterns. The patterns obtained after thedevelopment can be regarded as critical layer patterns on the wafer 20.However, it is also possible to carry out the following alignment andmeasurement of an amount of positional displacement between twocorresponding vernier marks with these marks left in the form of latentimages without effecting development.

[0306] Next, the second step will be explained.

[0307] A photoresist is coated over the wafer 20 having the wafer andvernier marks formed in the first step, and the photoresist-coated wafer20 is placed on the wafer stage 5B of the scanning exposure apparatus1B, shown in FIG. 29. At this time, information concerning the criticallayer shot map used in the first step has been supplied from thecontroller 10A of the fine stepper 1A to the controller 10B of thescanning exposure apparatus 1B. Thus, the controller 10B can obtaindesign array coordinates of the critical layer wafer and vernier markson the wafer 20.

[0308]FIG. 30(a) shows the wafer 20 placed on the wafer stage 5B. InFIG. 30(a), the X2- and Y2-axes of the scanning exposure apparatus 1Bare shown as being X- and Y-axes, respectively. In this case, the wafer20 has been roughly aligned by a pre-alignment mechanism (not shown),and the surface of the wafer 20 has been divided into Q (Q=32 in FIG.30(a)) critical layer shot areas SM1, SM2, . . . , SMQ in two directionswhich are approximately parallel to the directions X and Y,respectively. In actual practice, a scribe line area of a predeterminedwidth lies between shot areas SMq (q=1 to Q); however, illustration ofthe scribe line area is omitted in FIG. 30(a). The width (pitch) in thedirection X of each shot area SMq, including the scribe line area, is b,and the width (pitch) in the direction Y is 2 a. In this embodiment,each shot area SMq is approximately square (2a≈b). Further, each shotarea SMq is divided into two partial shot areas of the same shape in thedirection Y, that is, first and second partial areas 315A and 315B, inwhich circuit patterns identical with each other are to be formed.

[0309]FIG. 30(b) shows a shot area SMq as a typical example of thecritical layer shot areas. In FIG. 30(b), the first partial shot area inthe shot area SMq is provided with a pair of wafer marks 321XA and 321YAfor the X- and Y-axes, and also one set of four vernier marks 331A,331C, 331D and 331E which are distributed in a cross shape. Similarly,the second partial shot area in the shot area SMq is provided with apair of wafer marks 321XB and 321YB and four vernier marks 332A, 332C,332D and 332E in symmetric relation to the marks in the first partialshot area. In this case, the original drawing patterns of marksdistributed as shown in FIG. 30(b) have been formed in the pattern area62A of the reticle RA in the fine stepper 1A, shown in FIG. 29.

[0310] It should be noted that the wafer marks 321XA to 321YA, etc. usedin this example are one-dimensional line-and-space patterns which aredetected by an imaging detection method with the alignment system 11B,shown in FIG. 29. The vernier marks 331A to 331E, etc. aretwo-dimensional box-in-box marks which are detected by an imagingdetection method with the alignment system 11B. However, it is possibleto use other kinds of mark as vernier marks, for example, marks eachformed by a combination of two one-dimensional line-and-space patternswhich are crossed at right angles. It is also possible to use the wafermarks 321XA, 321YA, etc. themselves as vernier marks. Conversely,vernier marks may be used as wafer marks. In this embodiment, a part ofthe vernier marks 331A to 331E and 332A to 332E are used as multipointwafer marks (alignment marks) in the shot area SMq as one example.Further, marks which are detected by the laser step alignment (LEA)method, for example, may also be used as vernier marks. The distributionof wafer and vernier marks is not necessarily limited to that shown inFIG. 30(b).

[0311] Next, the controller 10B of the scanning exposure apparatus 1B,shown in FIG. 29, effects alignment by the EGA method. Accordingly, thecontroller 10B drives the wafer stage 5B to move the field of view ofthe alignment system 11B sequentially according to the critical layershot map, thereby measuring array coordinates in a stage coordinatesystem (i.e. a coordinate system determined by values measured with thelaser interferometers 7B and 9B of the scanning exposure apparatus 1B)of each of the wafer marks 321XA and 321YA attached to nine (forexample) shot areas (sample shots) S310, S320, . . . , S390 selectedfrom among the shot areas on the wafer 20, as shown in FIG. 30(a). Then,values of six EGA parameters (scaling parameters Rx and Ry, waferrotation Θ, shot perpendicularity W, and offsets ox and Oy) on the wafer20 are determined so as to minimize the residual error component, whichis the sum of the squares of deviations of the measured values of thewafer marks 321XA and 321YA of each sample shot from array coordinatevalues calculated from the design array coordinates of the wafer marks321XA and 321YA.

[0312] Next, the controller 10B determines array coordinate values ofeach shot area SMq (q=1 to Q) in the stage coordinate system from thesix EGA parameters and the design array coordinate values of thecritical layer shot area SMq. In this case, the exposure field 64B ofthe scanning exposure apparatus 1B is equal in size (1:1) to theexposure field 64A of the fine stepper 1A in the direction X but 3/2times as large as the exposure field 64A in the direction Y. Therefore,the controller 10B divides perfect partial shot areas (with no missingpart) in the shot areas SMq (q=1 to Q), shown in FIG. 30(a), into aplurality of blocks each comprising one partial shot area in thedirection X and three partial shot areas in the direction Y, each blockcontaining at least one shot area SMq. Then, the controller 10B obtainsarray coordinates of the center of each block in the stage coordinatesystem from the computational array coordinates of the shot area SMqcontained in the block. Thus, an array (shot map) of middle layer shotareas is determined.

[0313] For example, in the shot map for the middle layer to be exposedby the scanning exposure apparatus 1B, as shown in FIG. 31(a), R (R=20in FIG. 31(a)) shot areas SN1, SN2, . . . , SNR are arranged in thedirections X and Y over the critical layer on the wafer 20. The width(pitch) in the direction X of each shot area SNr (r=1 to R), includingthe scribe line area, is b, and the width (pitch) in the direction Y is3 a. Accordingly, assuming that the size of the middle layer shot areaSNr in the direction X is M₁/N₁ times as large as that of the criticallayer shot area SMq, and the size of the shot area SNr in the directionY is M₂/N₂ times as large as that of the shot area SMq, the size ratiosin this embodiment are M₁/N₁=1/1 and M₂/N₂=3/2. Further, each shot areaSNr is divided into three partial shot areas 316A to 316C of the samesize in the direction Y (i.e. the scanning direction). The three partialshot areas 316A to 316C are to be formed with identical patterns.

[0314] In this embodiment, it is assumed that six EGA parameters (Rx,Ry, Θ, W, Ox, and Oy) have no error, but four in-shot parameters (shotmagnifications rx and ry, shot rotation θ, and shot perpendicularity w)have errors. Therefore, overlay exposure is carried out in order toobtain errors (correction values) of these in-shot parameters.

[0315] That is, the scanning exposure apparatus 1B sequentiallytransfers an image of the vernier mark original drawing patterns formedon the reticle RB onto each of the middle layer shot areas SNr, shown inFIG. 31(a), by the scanning exposure method. Prior to the exposureprocess, the projection magnification and scanning speed of theprojection optical system 3B have been adjusted according to thecalculated shot magnifications rx and ry, and the reticle RB has beenrotated according to the shot rotation θ. Further, the scanningdirection has been adjusted according to the shot perpendicularity w.Thus, the middle layer chip pattern has previously been aligned withrespect to the critical layer chip pattern. After the exposure process,development is carried out, thereby allowing middle layer vernier marksto appear over the critical layer vernier marks on the wafer 20. Itshould be noted that the following measurement may be carried out withthe transferred marks left in the form of latent images, as has alreadybeen described above.

[0316] Next, the third step will be explained.

[0317] In the third step, measurement is carried out to determineamounts of positional displacement between the corresponding verniermarks in the critical layer shot areas SMq, shown in FIG. 30(a), and themiddle layer shot areas SNr, shown in FIG. 31(a). For this purpose, thewafer 20, shown in FIG. 31(a), which has been subjected to thedevelopment in the second step, is placed, for example, on the waferstage 5B of the scanning exposure apparatus 1B, shown in FIG. 29, andamounts of positional displacement between the corresponding verniermarks on the two layers are measured by the alignment system 11B.However, the measurement of positional displacement between thecorresponding vernier marks may be carried out by using anothermeasuring device of high accuracy.

[0318] In this case, it is assumed that, as shown in FIGS. 30(a) and31(a), the +Y direction end of the array of the critical layer shotareas SMq is coincident with the +Y direction end of the array of themiddle layer shot areas SNr. The shot areas SN1 to SNR of the middlelayer M are each provided with 12 (=4×3) vernier marks respectivelycorresponding to the vernier marks in the critical layer shot areas SMq(each having 8 vernier marks).

[0319]FIG. 31(b) shows a middle layer shot area SNr. In FIG. 31(b), thefirst partial shot area in the shot area SNr has four vernier marks 333Ato 333E formed so as to surround the critical layer vernier marks 331Ato 331E (see FIG. 30(b)), respectively. Similarly, the second and thirdpartial shot areas in the shot area SNr have four vernier marks 334A to334E and four vernier marks 335A to 335E, respectively, formed so as tosurround the corresponding critical layer vernier marks. In thisexample, it is assumed, for example, that the middle layer vernier mark335E is displaced by Δx and Δy in the directions X and Y relative to thevernier mark 331E in a predetermined critical layer shot area due toerrors of the four in-shot parameters (rx, ry, θ, and w). Accordingly,errors (correction values) of the in-shot parameters are obtained bydetecting amounts of positional displacement between the correspondingvernier marks on the two layers at predetermined measuring points.

[0320] A method of setting measuring points in two shot areas SNr andSN(r+1) which are contiguous with each other in the direction Y, asshown for example by the hatching in FIG. 31(a), will be explained belowwith reference to FIG. 32.

[0321] Referring to FIG. 32, areas in each of which one of the middlelayer shot areas SNr and SN(r+1) and one of the critical layer shotareas SMq, SM(q+1) and SM(q+2) are perfectly overlaid on one anothersuch that neither of the overlaid shot areas extends over a plurality ofmiddle or critical layer shot areas, that is, two hatched shot areas SMqand SM(q+2), are defined as reference measurement areas, and fourmeasuring points 336A to 336D are set in the first reference measurementarea SMq. Similarly, four measuring points 336E to 336H are set in thesecond reference measurement area SM(q+2) at respective positionscorresponding to the measuring points 336A to 336D.

[0322] At the measuring point 336A, amounts of positional displacementin the directions X and Y between the critical layer vernier mark 332Aand the middle layer vernier mark 334A are measured. Similarly, amountsof positional displacement between the corresponding vernier marks ofthe two layers at each of the other measuring points 336B to 336D and336E to 336H. It should be noted that other areas in FIG. 31(a) whereany one of the critical layer shot areas and any one of the middle layershot areas are perfectly overlaid on one another such that neither ofthe overlaid shot areas extends over a plurality of middle or criticallayer shot areas may be used as reference measurement areas in additionto the above.

[0323] Next, one example of a method obtaining errors of the fourin-shot parameters from the results of the measurement of amounts ofpositional displacement between two corresponding vernier marks at eachof the measuring points will be explained. Here, amounts of positionaldisplacement between the two corresponding vernier marks measured at twomeasuring points 336A and 336B, which are apart from each other in thedirection X in the first reference measurement area SMq, are denoted by(Δxa,Δya) and (Δxb,Δyb), and amounts of positional displacement betweenthe two corresponding vernier marks measured at two measuring points336C and 336D, which are apart from each other in the direction Y, aredenoted by (Δxc,Δyc) and (Δxd,Δyd). In this case, an error Δrx of the Xdirection shot magnification rx is obtained from the difference betweenthe amounts of positional displacement Δxa and Δxb, and an error Δry ofthe Y direction shot magnification ry is obtained from the differencebetween the amounts of positional displacement Δyc and Δyd. An error Δθof the shot rotation θ is obtained from the difference between theamounts of positional displacement Δya and Δyb. Further, an error Δw ofthe shot perpendicularity w is obtained from the difference between theamounts of positional displacement Δxc and Δxd and the shot rotationerror Δθ.

[0324] Further, mean values of in-shot parameter errors Δrx, Δry, Δθ andΔw, obtained in the other reference measurement areas, are determined,and these mean values are stored in the storage unit in the controller10B of the scanning exposure apparatus 1B as correction values Δrx′,Δry′, Δθ′ and Δw′ for the in-shot parameters. In this embodiment, noneof the reference measurement areas extend over two shot areas on eitherof the critical and middle layers. Therefore, the in-shot parametercorrection values obtained as described above are accurate values whichhave got rid of the effects of the stepping errors at the critical andmiddle layers.

[0325] In this regard, let us consider a case where, in FIG. 32, thecentral shot area SM(q+1), which extends over the two shot areas SNr andSN(r+1), is used as a reference measurement area, and measuring points337B and 337A are set in the two shot areas SNr and SN(r+1) within thereference measurement area. In this case, amounts of positionaldisplacement between the two corresponding vernier marks measured at thetwo measuring points 337B and 337A contain the middle layer steppingerror independently of each other. Therefore, even when an error of theshot magnification ry in the direction Y, for example, is calculatedfrom the sum of the amounts of positional displacement measured at thetwo points 337B and 337A, the calculated error contains the steppingerror. In other words, even if a reference measurement area extends overa plurality of shot areas of either layer, if a plurality of measuringpoints in the reference measurement area are set so that thedistribution of the measuring points does not extend over a plurality ofshot areas, the mixing of the stepping error can be prevented. In a casewhere amounts of positional displacement between the correspondingvernier marks are measured by another measuring device, and correctionvalues for the in-shot parameters are determined by another computer,for example, the operator inputs the correction values to the controller10B through an input unit or by on-line communication from thatcomputer. Thus, the third step is terminated.

[0326] In a case where exposure is carried out by the mix-and-matchmethod using the fine stepper 1A and the scanning exposure apparatus 1B,shown in FIG. 29, after the above-described third step, first, acritical layer pattern is formed on the wafer 20 by using the finestepper 1A. Thereafter, before exposure for a middle layer pattern iscarried out by using the scanning exposure apparatus 1B, coordinatepositions of a multiplicity of wafer marks in predetermined sample shotsare measured, and values of six wafer EGA parameters and four in-shotparameters are determined from the result of the measurement.Thereafter, the controller 10B adds the correction values (Δrx′, Δry′,Δθ′, and Δw′), stored in the above-described third step, to thedetermined in-shot parameters (rx, ry, θ, and w), thereby obtainingcorrected in-shot parameters. Then, the controller 10B calculates thecoordinate position of each shot area of the critical layer by using thesix wafer EGA parameters, calculates the exposure position for each shotarea of the middle layer on the basis of the calculated coordinatepositions, and sequentially effects positioning (e.g. setting of thescanning start position) of the middle layer shot areas on the basis ofthe calculated exposure positions. Then, the scanning exposure apparatus1B transfers an image of the reticle pattern onto each shot area by thescanning exposure method while correcting the image-formationcharacteristics according to the corrected in-shot parameter values. Inthis embodiment, the corrected in-shot parameter values are accurate;therefore, the overlay accuracy between the critical and middle layersis higher than in the conventional exposure process.

[0327] Next, other examples of the fourth embodiment of the presentinvention will be explained with reference to FIGS. 33(a) to 34(c). Inthe example shown in FIGS. 33(a) to 33(c), a critical layer shot area SKshown in FIG. 33(a) is allotted one chip pattern, and a middle layershot area SL shown in FIG. 33(b) is allotted a total of four identicalchip patterns arranged in an array of two columns in the direction X andtwo rows in the direction Y. The exposure apparatus for the criticallayer is a one-shot exposure type projection exposure apparatus(stepper) having a demagnification ratio of 5:1, and the exposureapparatus for the middle layer is a stepper having a demagnificationratio of 2.5:1.

[0328] Assuming that the width in the direction X of the critical layershot area SK is d, and the width in the direction Y of the shot area SKis c, the width in the direction X of the middle layer shot area SL is 2d, and the width in the direction Y of the shot area SL is 2 c.Therefore, the shot area SL is twice as large as the shot area SK ineach of the directions X and Y. Accordingly, as shown in FIG. 33(c), inan area 339 where an array of four critical layer shot areas and onemiddle layer shot area are overlaid on one another, an area where a shotarea SK and a shot area SL are overlaid on one another without extendingover a plurality of critical or middle layer shot areas, that is, eachcritical layer shot area SK itself, is used as a reference measurementarea. Therefore, two measuring points 340A and 340B are set in onereference measurement area 339 a, for example, and an amount ofpositional displacement between the corresponding vernier marks of thetwo layers is measured at each of the measuring points 340A and 340B. Bydoing so, correction values for in-shot parameters, e.g. the shotmagnification rx and the shot rotation θ, can be accurately obtained.

[0329] In the example shown in FIGS. 34(a) to 34(c), a first-layer shotarea SO shown in FIG. 34(a) is allotted a total of six identical chippatterns arranged in an array of two columns in the direction X andthree rows in the direction Y, and a second-layer shot area SP shown inFIG. 34(b) is allotted three identical chip patterns arranged in thedirection Y. The exposure apparatus for the first layer comprising theshot areas SO is a stepper, and the exposure apparatus for the secondlayer comprising the shot areas SP is a step-and-scan type projectionexposure apparatus.

[0330] Assuming that the width in the direction X of the shot area SO is2 b, and the width in the direction Y of the shot area SO is 3 a, thewidth in the direction X of the shot area SP is b, and the width in thedirection Y of the shot area SP is 3 a. That is, the shot area SP is½times as large as the shot area SO in the direction X, and the formeris equal in size (1:1) to the latter in the direction Y. According, asshown in FIG. 34(c), in an area 341 where one first-layer shot area andtwo second-layer shot areas are overlaid on one another, an area where ashot area SO and a shot area SP are overlaid on one another withoutextending over a plurality of first- or second-layer shot areas, thatis, each second-layer shot area SP itself, is used as a referencemeasurement area. Therefore, two measuring points 342A and 342B are setin one reference measurement area 341 a, for example, and an amount ofpositional displacement between the corresponding vernier marks of thetwo layers at each of the measuring points 342A and 342B. By doing so, acorrection value for an in-shot parameter, e.g. the shot magnificationry, can be accurately obtained.

[0331] Although in the above-described embodiment a combination of twosteppers or a combination of a stepper and a step-and-scan typeprojection exposure apparatus is used, it should be noted that thecombination of exposure apparatuses is not necessarily limited to thosedescribed above. For example, step-and-scan type projection exposureapparatuses which are different from each other may be used as twoexposure apparatuses having respective exposure fields of differentsizes.

[0332] According to the exposure method of the fourth embodiment, noneof the set reference measurement areas extend over a plurality of shotareas on either of two layers (e.g. critical and middle layers).Therefore, no stepping error is contained in an amount of positionaldisplacement between two corresponding overlay accuracy measuring marksmeasured at any of the measuring points in the reference measurementareas. Accordingly, the overlay accuracy between the two layers can beimproved by correcting the coordinates during alignment or theimage-formation characteristics on the basis of the measured amounts ofpositional displacement between the corresponding overlay accuracymeasuring marks.

[0333] Further, according to the fourth embodiment, an area where anyone of a plurality of first shot areas and any one of a plurality ofsecond shot areas are overlaid on one another such that neither of theoverlaid shot areas extends over beyond a part of that area (or neitherof them extends over a plurality of first or second shot areas) is usedas a reference measurement area, and an amount of positionaldisplacement between the corresponding overlay accuracy measuring marks(vernier marks) of the two layers is measured at each of measuringpoints set in predetermined reference measurement areas. Accordingly,correction values used in detection of the image positions of alignmentmarks (wafer marks) can be accurately obtained without being affected bystepping errors in the first and second shot areas. As a result, it ispossible to increase the overlay accuracy between a critical layerpattern and a middle layer pattern in a case where exposure is carriedout by the mix-and-match method with respect to a substrate where acritical layer and a middle layer are mixedly present. It is alsopossible to eliminate the effects of so-called seam errors between thefirst-layer shot areas and between the second-layer shot areas inaddition to the stepping error.

[0334] Further, it is possible to obtain a correction value for anin-shot parameter with high accuracy in a case where a correction valueobtained in the third step is a correction value for a parameterindicating a predetermined image-formation characteristic, which iscalculated on the basis of the positions of alignment mark images, andthe parameter indicating the predetermined image-formationcharacteristic is at least one parameter selected from the parametergroup consisting of shot magnification, shot rotation, and shotperpendicularity. Accordingly, the image-formation characteristics canbe corrected with high accuracy by using the corrected in-shotparameter.

[0335] In a case where the first exposure apparatus is a one-shotexposure type projection exposure apparatus, and the second exposureapparatus is a scanning exposure type projection exposure apparatus, theexposure method according to the present invention is particularlyeffective because in such a case the exposure fields of the two exposureapparatuses are likely to differ in size from each other. In the case ofa scanning exposure type projection exposure apparatus, the shotmagnification, shot rotation and shot perpendicularity can be readilycorrected at the time of exposure; therefore the overlay accuracybetween the two layers can be further increased by using parameterscorrected by the method according to the present invention.

[0336] It should be noted that the present invention is not necessarilylimited to the above-described first to fourth embodiments, but mayadopt various arrangements without departing from the gist of thepresent invention.

1. An exposure method in which mask patterns are overlaid on one anotheron a photosensitive substrate, which is an object to be exposed, byusing a first exposure apparatus having a first exposure field of apredetermined size on said photosensitive substrate, and a secondexposure apparatus having a second exposure field which is M₁/N₁ times(M₁ and N₁ are integers; M₁>N₁) as large as said first exposure field ina first direction and which is M₂/N₂ times (M₂ and N₂ are integers;M₂≧N₂) as large as said first exposure field in a second direction whichis perpendicular to said first direction, said exposure methodcomprising: a first step of sequentially transferring an image of afirst mask pattern, which has an alignment mark and a first overlayaccuracy measuring mark, onto said photosensitive substrate in the formof a two-dimensional array extending in said first and second directionsin units of said first exposure field by using said first exposureapparatus; a second step of transferring an image of a second maskpattern, which has a second overlay accuracy measuring mark, over aplurality of images of said first mask pattern which have beentransferred onto said photosensitive substrate in said first step, in atwo-dimensional array extending in said first and second directions onsaid photosensitive substrate in units of said second exposure fieldwith reference to a position of an image of said alignment mark by usingsaid second exposure apparatus; and a third step of dividing an exposurearea on said photosensitive substrate into a plurality of referencemeasurement areas in units of an area which is N₁ times as large as awidth of said second exposure field in said first direction on saidphotosensitive substrate and which is N₂ times as large as a width ofsaid second exposure field in said second direction on saidphotosensitive substrate, and measuring an amount of positionaldisplacement between images of said first and second overlay accuracymeasuring marks lying at mutually identical positions in a predeterminednumber of reference measurement areas selected from among said pluralityof reference measurement areas, thereby obtaining a correction valuewhich is used when the position of the image of said alignment marktransferred by said first exposure apparatus is detected by said secondexposure apparatus on the basis of said measured amount of positionaldisplacement; wherein an exposure position is corrected by using saidcorrection value obtained in said third step when overlay exposure is tobe carried out thereafter by using said second exposure apparatus withrespect to a surface of said photosensitive substrate exposed by saidfirst exposure apparatus.
 2. An exposure method according to claim 1,wherein, in said second step, said second exposure apparatus calculatesan exposure position on the basis of the image of said alignment markand by use of a predetermined coordinate transformation parameter, andin said third step, said second exposure apparatus obtains a correctionvalue for said coordinate transformation parameter.
 3. An exposuremethod in which mask patterns are overlaid on one another on aphotosensitive substrate by using a first exposure apparatus and asecond exposure apparatus having respective exposure fields of differentsizes, said exposure method comprising the steps of: sequentiallytransferring images of first and second mask patterns containing overlayaccuracy measuring marks onto a photosensitive substrate for evaluationsuch that the images of said first and second mask patterns are overlaidon one another by using said first and second exposure apparatus;measuring an amount of positional displacement between the overlaidimages of said overlay accuracy measuring marks at a predeterminedmeasuring point in a reference measurement area on said evaluationphotosensitive substrate in which a shot area formed in units of theexposure field of said first exposure apparatus and a shot area formedin units of the exposure field of said second exposure apparatus areoverlaid on one another such that neither of said overlaid shot areasextends beyond a part of said reference measurement area; and effectingalignment or correction of image-formation characteristics on the basisof a result of said measurement when exposure is to be carried out bysaid second exposure apparatus with respect to a surface of saidphotosensitive substrate exposed by said first exposure apparatus.
 4. Anexposure method in which mask patterns are overlaid on one another on aphotosensitive substrate, which is an object to be exposed, by using afirst exposure apparatus having a first exposure field of apredetermined size on said photosensitive substrate, and a secondexposure apparatus having a second exposure field which is M₁/N₁ times(M₁ and N₁ are integers; M₁≠N₁) as large as said first exposure field ina first direction and which is M₂/N₂ times (M₂ and N₂ are integers) aslarge as said first exposure field in a second direction which isperpendicular to said first direction, said exposure method comprising:a first step of sequentially transferring an image of a first maskpattern, which has an alignment mark and a first overlay accuracymeasuring mark, onto a plurality of first shot areas arrayed on saidphotosensitive substrate in units of said first exposure field by usingsaid first exposure apparatus; a second step of sequentiallytransferring an image of a second mask pattern, which has a secondoverlay accuracy measuring mark, onto a plurality of second shot areasarrayed on said photosensitive substrate, exposed in said first step, inunits of said second exposure field with reference to a position of animage of said alignment mark by using said second exposure apparatus;and a third step of defining a plurality of reference measurement areason said photosensitive substrate in each of which any one of said firstshot areas and any one of said second shot areas are overlaid on oneanother such that neither of said overlaid shot areas extends beyond apart of said reference measurement area, and measuring an amount ofpositional displacement between images of said first and second overlayaccuracy measuring marks lying at mutually identical positions in apredetermined number of reference measurement areas selected from amongsaid plurality of reference measurement areas, thereby obtaining acorrection value which is used when the position of the image of saidalignment mark transferred by said first exposure apparatus is detectedby said second exposure apparatus on the basis of said measured amountof positional displacement.
 5. An exposure method according to claim 4,wherein the correction value obtained in said third step is a correctionvalue for a parameter indicating a predetermined image-formationcharacteristic calculated on the basis of the position of the image ofsaid alignment mark, said parameter being at least one parameterselected from a parameter group consisting of shot magnification, shotrotation, and shot perpendicularity, and wherein said image-formationcharacteristic is corrected by using the correction value obtained insaid third step when overlay exposure is to be carried out thereafter byusing said second exposure apparatus with respect to a surface of saidphotosensitive substrate exposed by said first exposure apparatus.
 6. Anexposure method according to claim 4, wherein said first exposureapparatus is a one-shot exposure type projection exposure apparatus, andsaid second exposure apparatus is a scanning exposure type projectionexposure apparatus.
 7. An exposure method in which a substrate isexposed with a second pattern by using a second exposure apparatus, themethod comprising: providing the substrate on which a plurality of firstshot areas are formed, the plurality of first shot areas on thesubstrate being formed by exposing the substrate with a first pattern byusing a first exposure apparatus before exposing the substrate with thesecond pattern, each of the first shot areas having M partial areasalong a first direction (M is an integer), and the first and secondexposure apparatus having respective exposure fields of different sizes;and performing a scanning exposure in which each of a plurality ofsecond shot areas is exposed while moving the substrate along the firstdirection in the second exposure apparatus, each of the second shotareas having N partial areas along the first direction (N is an integer;N≠M), each partial area of the first shot areas substantiallyoverlapping with each partial area of the second shot areas.
 8. Anexposure method according to claim 7, wherein the substrate is exposedby using the first exposure apparatus so that the first shot areasformed into a plurality of rows parallel to the first direction and thefirst shot areas adjacent to each other in the first direction arearranged without positional deviation in a second directionperpendicular to the first direction.
 9. An exposure method according toclaim 7, wherein a position of the substrate in the second directionduring scanning exposure for one second shot area SC1 is different froma position of the substrate in the second direction during scanningexposure for another second shot area SC2 which is adjacent to the onesecond shot area SCI in the first direction.
 10. An exposure methodaccording to claim 9, further comprising; obtaining shot rotationinformation θ of each of the plurality of first shot areas, wherein thesubstrate is moved in the first direction based on the obtained shotrotation information θ.
 11. An exposure method according to claim 7,further comprising: obtaining shot rotation information θ of each of theplurality of first shot areas, wherein the substrate is moved in thefirst direction based on the obtained shot rotation information θ. 12.An exposure method according to claim 7, wherein M is an even number andN is an odd number.